android布局看不到activity.kt文件，需要java文件

这是布局：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">



            <LinearLayout
                android:id="@+id/linearLayout"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginStart="4dp"
                android:layout_marginTop="2dp"
                android:layout_marginEnd="4dp"
                android:orientation="horizontal"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent">

                <Button
                    android:id="@+id/btn_text"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:layout_weight="1"
                    android:text="Generate QR"
                    android:textAllCaps="false" />

            </LinearLayout>

            <RelativeLayout
                android:id="@+id/relativeLayout"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginStart="4dp"
                android:layout_marginTop="2dp"
                android:layout_marginEnd="4dp"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/linearLayout">



                <EditText
                    android:id="@+id/input_text"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:ems="10"
                    android:inputType="text"
                    android:hint="Enter text here"
                    android:visibility="gone"

                    />

            </RelativeLayout>
            <Button
                android:id="@+id/btn_generateQR"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="Generate QR code"
                android:textAllCaps="false"
                android:visibility="gone"/>


            <ImageView
                android:id="@+id/imageView_qrCode"
                android:layout_gravity="center"
                android:layout_width="250dp"
                android:layout_height="250dp"
                android:layout_marginBottom="2dp"
                android:layout_marginTop="2dp"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/relativeLayout"
                />

            <Button
                android:id="@+id/btn_save"
                android:layout_width="200dp"
                android:layout_height="wrap_content"
                android:text="Save to gallery"
                android:textAllCaps="false"
                android:layout_gravity="center"
                android:visibility="gone"/>
        </LinearLayout>
    </ScrollView>


</androidx.constraintlayout.widget.ConstraintLayout>

这是激活：

class MainActivity : AppCompatActivity(), View.OnClickListener {

    private var qrImage : Bitmap? = null
    val EXTERNAL_STORAGE_PERMISSION_REQUEST_CODE = 1

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        btn_text.setOnClickListener(this)
        btn_save.setOnClickListener(this)
        btn_generateQR.setOnClickListener(this)


        if (!checkPermissionForExternalStorage()) {
            requestPermissionForExternalStorage()
        }
    }

    override fun onClick(v: View?) {
        when(v?.id)
        {
            R.id.btn_text->
            {
                generateQRCode()
            }
        }
    }

    fun generateQRCode()
    {
        val newUUID = UUID.randomUUID()
        qrImage = net.glxn.qrgen.android.QRCode.from(newUUID.toString()).bitmap()
        if(qrImage != null) {
            var reqParam = URLEncoder.encode("qr", "UTF-8") + "=" + URLEncoder.encode(newUUID.toString(), "UTF-8") + "&"
            var session = URLEncoder.encode("session", "UTF-8") + "=" + URLEncoder.encode(Data.responseBody, "UTF-8")
            AsyncRequest().execute("/addHash", reqParam, session)
            imageView_qrCode.setImageBitmap(qrImage)
            btn_save.visibility = View.VISIBLE
        }
    }

    private fun requestPermissionForExternalStorage() {
        if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
            Toast.makeText(this, "External Storage permission needed. Please allow in App Settings for additional functionality.", Toast.LENGTH_LONG).show()
        } else {
            ActivityCompat.requestPermissions(this, arrayOf<String>(Manifest.permission.WRITE_EXTERNAL_STORAGE), EXTERNAL_STORAGE_PERMISSION_REQUEST_CODE)
        }
    }

    private fun checkPermissionForExternalStorage(): Boolean {

        val result = ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE)
        return result == PackageManager.PERMISSION_GRANTED
    }
}

就行了错误：tools:context=".MainActivity 按照建议创建文件的时候，MainActivity.java被创建了，告诉我设置里面需要改什么