Android Mediaplayer multiple instances when activity resumes

Question

I am developing an app that plays a stream by using the built-in Android Mediaplayer. Everything works fine apart the fact that when I press the "home" button and then open the activity again, the app just starts a new instance of the mediaplayer and I can hear the stream twice without the possibility to stop the first one.

Scenario.

My app is only made of one activity.

1. I open my app first time and the stream starts.
2. Push the home button and the activity is "minimized" but the music still plays
3. I open my app again and it just starts a new instance of the mediaplayer playing the same music twice.

I check onResume if the mediaplayer is null or playing but somehow it appears to be always null thus a new instance is created.

My pseudo code...

public class MainActivity {

private MediaPlayer _mp;

private PlayTask _playTask;
private PauseTask _pauseTask;

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

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

    if(_mp != null && _mp.isPlaying()){
        // I should find a playing mediaplayer here but it's always null!!!
        Toast.makeText(MainActivity.this, "Playing mediaplayer found!", Toast.LENGTH_SHORT).show();
    }
    else{
        Toast.makeText(MainActivity.this, "No mediaplayer found :(", Toast.LENGTH_SHORT).show();
    }
}

/* ...:::============= *** ASYNCTASK Definition START *** =============:::... */

private class PlayTask extends AsyncTask<Void, Void, Integer> {

    @Override
    protected void onPreExecute() {
        if(_mp == null)
                _mp = new MediaPlayer();
            else if(_mp.isPlaying()){
                _mp.stop();
                _mp.release();
                _mp = null;
                _mp = new MediaPlayer();
            }
    }

    @Override
    protected Integer doInBackground(Void...voids) {
        try{
            _mp.setDataSource(URL_DATASOURCE);
            _mp.setAudioStreamType(AudioManager.STREAM_MUSIC);
            _mp.prepare();
            _mp.start();
        }
        catch(Exception ex){
        }
        return 0;
    }

    @Override
    protected void onPostExecute(Integer i) {

    }
}

private class PauseTask extends AsyncTask<Void, Void, Integer> {

    @Override
    protected void onPreExecute() {
        // Do Nothing here...for now
    }

    @Override
    protected Integer doInBackground(Void...voids) {
        _mp.stop();
        _mp.release();
        _mp = null;
        return 0;
    }

    @Override
    protected void onPostExecute(Integer i) {
    }
}


public void playPause(View view){
    if(_play){
        _playTask = new PlayTask();
        _playTask.execute();
    }
    else{
        _pauseTask = new PauseTask();
        _pauseTask.execute();
    }
}

}

Any idea? Thanks