Android 使用 AsyncTask 后报错

有以下片段：package tanat.androidtesttask.fragments；

import android.content.BroadcastReceiver;
import android.content.ContentValues;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.database.sqlite.SQLiteDatabase;
import android.os.AsyncTask;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.RequiresApi;
import android.support.v4.widget.SwipeRefreshLayout;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.TextView;

import com.google.firebase.crash.FirebaseCrash;

import java.util.ArrayList;

import butterknife.BindView;
import butterknife.ButterKnife;
import butterknife.OnClick;

import io.realm.Realm;
import io.realm.RealmAsyncTask;
import io.realm.RealmConfiguration;
import io.realm.RealmResults;
import tanat.androidtesttask.BuildConfig;
import tanat.androidtesttask.activity.InfoRoutActivity;
import tanat.androidtesttask.R;
import tanat.androidtesttask.activity.MainActivity;
import tanat.androidtesttask.database.DBHelper;
import tanat.androidtesttask.database.RealmController;
import tanat.androidtesttask.model.RealmModel;
import tanat.androidtesttask.service.BroadcastService;
import tanat.androidtesttask.utils.JSONParsing;
import tanat.androidtesttask.utils.LoadLocalData;
import tanat.androidtesttask.errorreporter.Log;

public class ListFragment extends android.app.ListFragment implements SwipeRefreshLayout.OnRefreshListener{

    @BindView(R.id.refresh) SwipeRefreshLayout swipeRefreshLayout;
    @BindView(R.id.standart_layout) LinearLayout contentLayout;
    @BindView(R.id.error_layout) LinearLayout errorLayout;
    @BindView(R.id.errorTextView) TextView errorTextView;

    //button to update if an error
    @OnClick(R.id.refreshButton)
    void onRefreshClick() {
        onRefresh();
    }

    private View rootView;

    private final static int STATUS_START = 100;
    private final static int STATUS_FINISH = 200;
    private final static String PARAM_RESULT = "result";
    private final static String PARAM_STATUS = "status";
    public final static String BROADCAST_ACTION = "tanat.androidtesttask.activity";
    private BroadcastReceiver broadcastReceiver;

    private LoadLocalData loadLocalData;
    DBHelper dbHelper;
    Realm realm;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        rootView = inflater.inflate(R.layout.fragment_item, null);
        ButterKnife.bind(this, rootView);

        swipeRefreshLayout.setOnRefreshListener(this);

        intent = new Intent(getActivity(), BroadcastService.class);
        setsConnection();

        loadLocalData = new LoadLocalData(getActivity());

        dbHelper = new DBHelper(getActivity());

        Realm.init(getActivity());

        setRetainInstance(true);
        return rootView;
    }

    Intent intent;
    private ArrayList data = null;

    private void setsConnection (){
        broadcastReceiver = new BroadcastReceiver() {
            public void onReceive(Context context, Intent intent) {
                int status = intent.getIntExtra(PARAM_STATUS, 0);
                if (status == STATUS_START) {
                    if (BuildConfig.USE_LOG) {Log.d("server start task");}
                    FirebaseCrash.log("server start task");
                }
                if (status == STATUS_FINISH) {
                    if (BuildConfig.USE_LOG) {Log.d("server finish task");}
                    FirebaseCrash.log("server finish task");

                    data = intent.getStringArrayListExtra(PARAM_RESULT);
                    procesShowData();

                    saveCache();
                }
            }
        };
        IntentFilter intFilt = new IntentFilter(BROADCAST_ACTION);

        getActivity().registerReceiver(broadcastReceiver, intFilt);
    }

    @Override
    public void onStart() {
        super.onStart();
        loadData();
        procesShowData();
    }

    @Override
    public void onPause(){
        // save the data so that you do not load it again when return to the fragment
        if(data != null && !data.get(0).toString().equals("false")) {
            if (BuildConfig.USE_LOG) {Log.d("pause before save");}
            //save data in file
            loadLocalData.writeFile(FILE_NAME, new JSONParsing().dispatch());
        }
        if (BuildConfig.USE_LOG) {Log.d("pause last save");}
        super.onPause();
    }

    @Override
    public void onDestroy() {
        getActivity().unregisterReceiver(broadcastReceiver);
        getActivity().stopService(intent);
        super.onDestroy();
    }

    @RequiresApi(api = Build.VERSION_CODES.M)
    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        super.onListItemClick(l, v, position, id);
        loadLocalData.writeFile(FILE_NAME, new JSONParsing().dispatch());

        Intent intent = new Intent(ListFragment.this.getContext(), InfoRoutActivity.class);
        intent.putExtra("position", position);
        startActivity(intent);
    }

    @Override
    public void onRefresh() {
        swipeRefreshLayout.setRefreshing(true);

        getActivity().startService(intent);
    }

    private String FILE_NAME = "JsonTestTask";
    // loading data
    private void loadData (){
        // start show progress dialog
        swipeRefreshLayout.setRefreshing(true);

        // upload local data
        data = loadLocalData.returnArray(FILE_NAME);

        //if there is no data, load it from the network
        if (data == null || data.size() == 0 || data.get(0).toString().equals("false")) {
            data = null;
            getActivity().startService(intent);
        }
    }

    private void procesShowData (){
        if(data != null){
            if(data.get(0).toString().equals("false")){
                contentLayout.setVisibility(View.INVISIBLE);
                errorLayout.setVisibility(View.VISIBLE);
                errorTextView.setText(data.get(1).toString());
            } else {
                // if network operation true
                errorLayout.setVisibility(View.INVISIBLE);
                contentLayout.setVisibility(View.VISIBLE);
                // create list
                createdList();
            }
        } else {
            errorLayout.setVisibility(View.INVISIBLE);
            contentLayout.setVisibility(View.VISIBLE);
            createdList();
        }
        if (data != null) {
            swipeRefreshLayout.setRefreshing(false);
        }
    }

    private ArrayAdapter<String> adapter;

    // create list
    public void createdList(){
        if (data != null){
            adapter = new ArrayAdapter<String>(getActivity(),
                    android.R.layout.simple_list_item_1, data);
        } else {
            adapter = null;
        }
        setListAdapter(adapter);
    }

    public void saveCache (){
        int checkTypeDatabase = new MainActivity().checkTypeDatabase;
        if (checkTypeDatabase == 1) {
            // use Realm database
            CacheRealm cacheRealm = new CacheRealm();
            cacheRealm.execute();

        } else if (checkTypeDatabase == 2) {
            // use SQLite database
            CacheSQLite cacheSQLite = new CacheSQLite();
            cacheSQLite.execute();
        }
    }

    private int id = 0;

    private int name_from_city = 1;
    private int highlight_from_city = 2;
    private int id_from_city = 3;

    private int name_to_city = 4;
    private int highlight_to_city = 5;
    private int id_to_city = 6;

    private int info = 7;

    private int from_date = 8;
    private int from_time = 9;
    private int from_info = 10;

    private int to_date = 11;
    private int to_time = 12;
    private int to_info = 13;

    private int price = 14;
    private int bus_id = 15;
    private int reservation_count = 16;

    // Caching the query in the Realm database
    class CacheRealm extends AsyncTask<Void, Void, Void> {

        @Override
        protected Void doInBackground(Void... params) {
            ArrayList<String[]> arrayList = new JSONParsing().examineJSONObj();

            Realm realm = Realm.getDefaultInstance();

            try {
            realm.beginTransaction();

            RealmResults<RealmModel> results = realm.where(RealmModel.class).findAll();
            results.deleteAllFromRealm();

            for (int i = 0; i < arrayList.size(); i++) {
                RealmModel realmObject = realm.createObject(RealmModel.class);

                realmObject.setId(Integer.valueOf(arrayList.get(i)[id]));

                realmObject.setName_from_city(arrayList.get(i)[name_from_city]);
                realmObject.setHighlight_from_city(arrayList.get(i)[highlight_from_city]);
                realmObject.setId_from_city(Integer.valueOf(arrayList.get(i)[id_from_city]));

                realmObject.setName_to_city(arrayList.get(i)[name_to_city]);
                realmObject.setHighlight_to_city(arrayList.get(i)[highlight_to_city]);
                realmObject.setId_to_city(Integer.valueOf(arrayList.get(i)[id_to_city]));

                realmObject.setInfo(arrayList.get(i)[info]);

                realmObject.setFrom_date(arrayList.get(i)[from_date]);
                realmObject.setFrom_time(arrayList.get(i)[from_time]);
                realmObject.setFrom_info(arrayList.get(i)[from_info]);

                realmObject.setTo_date(arrayList.get(i)[to_date]);
                realmObject.setTo_time(arrayList.get(i)[to_time]);
                realmObject.setTo_info(arrayList.get(i)[to_info]);

                realmObject.setPrice(Integer.valueOf(arrayList.get(i)[price]));
                realmObject.setBus_id(Integer.valueOf(arrayList.get(i)[bus_id]));
                realmObject.setReservation_count(Integer.valueOf(arrayList.get(i)[reservation_count]));
            }

            realm.commitTransaction();

            } finally {
                realm.close();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            if (BuildConfig.USE_LOG) {Log.d("The cache is written to the Realm database");}
            super.onPostExecute(result);
        }
    }

    // Caching the query in the SQLite database
    class CacheSQLite extends AsyncTask<Void, Void, Void> {

        @Override
        protected Void doInBackground(Void... params) {
            SQLiteDatabase db = dbHelper.getWritableDatabase();

            ContentValues values = new ContentValues();
            ArrayList<String[]> arrayList = new JSONParsing().examineJSONObj();

            db.delete(dbHelper.TABLE_NAME, null, null);
            for (int i = 0; i < arrayList.size(); i++){
                values.put(dbHelper.ID, arrayList.get(i)[id]);
                values.put(dbHelper.NAME_FROM_CITY, arrayList.get(i)[name_from_city]);
                values.put(dbHelper.HIGHLIGHT_FROM_CITY, arrayList.get(i)[highlight_from_city]);
                values.put(dbHelper.ID_FROM_CITY, arrayList.get(i)[id_from_city]);
                values.put(dbHelper.NAME_TO_CITY, arrayList.get(i)[name_to_city]);
                values.put(dbHelper.HIGHLIGHT_TO_CITY, arrayList.get(i)[highlight_to_city]);
                values.put(dbHelper.ID_TO_CITY, arrayList.get(i)[id_to_city]);
                values.put(dbHelper.INFO, arrayList.get(i)[info]);
                values.put(dbHelper.FROM_DATE, arrayList.get(i)[from_date]);
                values.put(dbHelper.FROM_TIME, arrayList.get(i)[from_time]);
                values.put(DBHelper.FROM_INFO, arrayList.get(i)[from_info]);
                values.put(DBHelper.TO_DATE, arrayList.get(i)[to_date]);
                values.put(dbHelper.TO_TIME, arrayList.get(i)[to_time]);
                values.put(dbHelper.TO_INFO, arrayList.get(i)[to_info]);
                values.put(dbHelper.PRICE, arrayList.get(i)[price]);
                values.put(dbHelper.BUS_ID, arrayList.get(i)[bus_id]);
                values.put(dbHelper.RESERVATION_COUNT, arrayList.get(i)[reservation_count]);

                db.insert(dbHelper.TABLE_NAME, null, values);
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            if (BuildConfig.USE_LOG) {Log.d("The cache is written to the SQLite database");}
            super.onPostExecute(result);
        }
    }
}

问题是，如果您在使用其中一个 AsyncTask 之后调用 onPause 方法（即使在最后一个完成之后）。错误文本如下：

    02-01 03:52:39.581 1688-1688/tanat.androidtesttask E/UncaughtException: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.FileOutputStream android.content.Context.openFileOutput(java.lang.String, int)' on a null object reference
                                                                                at android.content.ContextWrapper.openFileOutput(ContextWrapper.java:183)
at tanat.androidtesttask.d.b.a(Unknown Source)
at tanat.androidtesttask.fragments.ListFragment.onListItemClick(Unknown Source)
at android.app.ListFragment$2.onItemClick(ListFragment.java:160)
at android.widget.AdapterView.performItemClick(AdapterView.java:310)
at android.widget.AbsListView.performItemClick(AbsListView.java:1145)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:3042)
at android.widget.AbsListView.onTouchUp(AbsListView.java:3891)
at android.widget.AbsListView.onTouchEvent(AbsListView.java:3656)
at android.view.View.dispatchTouchEvent(View.java:9294)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2547)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2240)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at com.android.internal.policy.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2403)
at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1737)
at android.app.Activity.dispatchTouchEvent(Activity.java:2765)
at com.android.internal.policy.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2364)
at android.view.View.dispatchPointerEvent(View.java:9514)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4230)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4096)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3695)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3661)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3787)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3669)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3844)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3695)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3661)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3669)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3642)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5922)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5896)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5857)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6025)
at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
at android.os.MessageQueue.nativePollOnce(Native Method)
at android.os.MessageQueue.ne
    02-01 03:52:39.586 1688-1688/tanat.androidtesttask E/AndroidRuntime: FATAL EXCEPTION: main
Process: tanat.androidtesttask, PID: 1688
java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.FileOutputStream android.content.Context.openFileOutput(java.lang.String, int)' on a null object reference
at android.content.ContextWrapper.openFileOutput(ContextWrapper.java:183)
at tanat.androidtesttask.d.b.a(Unknown Source)
at tanat.androidtesttask.fragments.ListFragment.onListItemClick(Unknown Source)
at android.app.ListFragment$2.onItemClick(ListFragment.java:160)
at android.widget.AdapterView.performItemClick(AdapterView.java:310)
at android.widget.AbsListView.performItemClick(AbsListView.java:1145)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:3042)
at android.widget.AbsListView.onTouchUp(AbsListView.java:3891)
at android.widget.AbsListView.onTouchEvent(AbsListView.java:3656)
at android.view.View.dispatchTouchEvent(View.java:9294)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2547)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2240)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2553)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2254)
at com.android.internal.policy.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2403)
at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1737)
at android.app.Activity.dispatchTouchEvent(Activity.java:2765)
at com.android.internal.policy.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2364)
at android.view.View.dispatchPointerEvent(View.java:9514)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4230)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4096)
at 
android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
at android.os.MessageQueue.

我不明白问题出在哪里，因为没有一个 asyncTasks 使用 onPause 使用的任何东西。请帮助理解。