Android : Move activity to front in AsyncTask

In an activity, I have created a AsyncTask after hiding the activity:

this.moveTaskToBack(true);
(new MyTask(this)).execute();

To show a dialog in the task (in onPostExcecute), I want to bring the activity to front:

alertDialog.show();

Intent intent = new Intent(mainActivity, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
mainActivity.getBaseContext().startActivity(intent);

But the main activity is shown on top of the dialog (the activity has also a dialog style Theme.Dialog). How should I fix this?