Activity seems to finish directly on 2nd resume

Question

Let's pretend I have the activities MyActivityA and MyActivityB.

1. MyActivityA starts MyActivityB
2. MyActivityB starts an repeating AlarmManager who triggers an IntentService which is informing MyActivityB with a ResultReceiver when something happens.
3. something happened, MyActivityB opens an AlertDialog, the PositiveButton leads to MyActivityA

(IntentService makes GET-Requests, something is an expected GET-Response)

The first time it's working. But when I then repeat step one and two, the AlertDialog does not appear and the app crashes with BadTokenException: Unable to add window -- token ... is not valid; is your activity running?

A check at step three showed, that isFinishing() is true the second time. That explains the error. But why is the activity finishing right after getting resumed?

I try to give you the relevant part of the code:

MyActivityA:

public class MyActivityA extends AppCompatActivity{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my_a);
    }

    public void sendButtonPressed(View view) {
        Intent intent = new Intent(MyActivityA.this, MyActivityB.class);
        startActivity(intent);
    }
}

MyActivityB:

public class MyActivityB extends AppCompatActivity implements CheckIPResultReceiver.Receiver{

    private CheckIPResultReceiver mReceiver;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my_b);

        mReceiver = new CheckIPResultReceiver(new Handler());
        mReceiver.setReceiver(this);
    }

    @Override
    protected void onResume() {
        super.onResume();

        //starting repeated IntentService via AlarmManager
        String targetURL = "www.google.de";
        Intent alarmIntent = new Intent(this, AlarmReceiver.class);
        alarmIntent.putExtra("targetURL", targetURL);

        PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 0, alarmIntent, 0);
        AlarmManager alarmManager=(AlarmManager) getSystemService(ALARM_SERVICE);
        alarmManager.setRepeating(AlarmManager.RTC_WAKEUP, System.currentTimeMillis(), 10000, pendingIntent);
    }

    public void onReceiveResult(int resultCode, Bundle resultData) {

        //getting events from the service
        switch (resultCode) {
            case SendPostRequest_Service.STATUS_ALERT:

                //stopping AlarmManager
                Intent alarmIntent = new Intent(MyActivityB.this, AlarmReceiver.class);
                PendingIntent pendingIntent = PendingIntent.getBroadcast(MyActivityB.this, 0, alarmIntent, 0);
                AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
                alarmManager.cancel(pendingIntent);

                //debug-check
                if(isFinishing()){
                    // on the 2nd time --> TRUE!
                    Log.v(TAG, "Act. seems to finish?!");
                }
                else {
                    // only on the 1st time true
                    Log.v(TAG, "Act. seems ok");
                    new AlertDialog.Builder(MyActivityB.this)
                            .setTitle("Alert triggered!")
                            .setMessage("...")
                            .setCancelable(false)
                            .setPositiveButton("Restart", new DialogInterface.OnClickListener() {
                                public void onClick(DialogInterface dialog, int which) {
                                    Intent intent = new Intent(MyActivityB.this, MyActivityA.class);
                                    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                                    startActivity(intent);
                                    finish();
                                }
                            })
                            .setIcon(android.R.drawable.ic_dialog_alert)
                            .show();
                }
                break;
        }
    }
}

Do you have any suggestions? I don't find any reason why MyActivityB goes into finishing...