libgdx: unlocked Achievement pop-up not shown

Question

Some users in this site asked my question like this unlocked-popup-not-showing and Unlocked Achievement Popup not shown but doesn't work for me I don't know what is wrong in my code?

Firstly to clear my question for all users. My Question is How can I show a pop-up notification for unlocked Achievement? like this image:

Secondly my android class MainActivity implements the following:

public class MainActivity extends AndroidApplication implements IGoogleServices, GameHelperListener {
     private GameHelper _gameHelper;
.......

which IGoogleServices interface is:

public interface IGoogleServices {
    .....
    public boolean isSignedIn();
    public void unlockAchievement(String achievementId);
    public void showAchievements();
}

Thirdly the code in onCreate method:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    AndroidApplicationConfiguration config = new AndroidApplicationConfiguration();

    // Create the GameHelper.
    _gameHelper = new GameHelper(this, GameHelper.CLIENT_GAMES);
    _gameHelper.enableDebugLog(true);

    _gameHelper.setup(this);

    initialize(new MyGdxGame(this, this), config);
}

Fourthly the implementaion methods:

@Override
public void unlockAchievement(String achievementId) {
    if (isSignedIn()) {
        Games.Achievements.unlock(_gameHelper.getApiClient(), achievementId);
    }
}

Fifthly I threw the IGoogleServices interface into MyGdxGame game:

public MyGdxGame(IGoogleServices googleServices) {
    this.googleServices = googleServices;
}

Finally I created Image as button to unlock the first achievement:

final Image iAch1 = new Image(ach1);
iAch1.addListener(new InputListener() {
    public boolean touchDown(InputEvent event, float x, float y, int pointer, int button) {
        return true;
    }
    public void touchUp(InputEvent event, float x, float y, int pointer, int button) {
        googleServices.unlockAchievement("CgkIzZ.......");
    }
});
stage.addActor(iAch1);

So far everything is okay and the Achievements unlocked BUT without pop-up notifications as shown in previous image. How can I achieve that?