如何在 alertDialog 中填充数据微调器

java.lang.NullPointerException我在尝试分配adapter spinner-y时偶然发现。很明显，我是在还没有打开spinner的时候尝试填写数据。alertDialog告诉我何时何地需要正确分配adapter spinner-y。

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {


public AlertDialog dialog;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);



    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    Spinner spinner;
    spinner = (Spinner) findViewById(R.id.spinner);
    ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_spinner_item, budgetItems);
    dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    //тут ловлю Null
    spinner.setAdapter(dataAdapter);

    this.dialog = builder.create();
    this.dialog.setView(this.getLayoutInflater().inflate(R.layout.dialog_add_item, null, true));
    }
}