WidgetGroup + Actions.fadeOut = Black Screen

Problem:
          WidgetGroup: addAction(Actions.fadeOut(2f));

          

Solution:

     @Override      

     public void draw(Batch batch, float parentAlpha) {  
         super.draw(batch, parentAlpha);
         batch.setColor(this.getColor());
         batch.getColor().a *= parentAlpha;

         batch.setColor(Color.WHITE);
     }     

            
Explanation:
          Don't know (need your helps)