protected override void OnPause()
 {
     base.OnPause();
     ModalActivityManager.ClearReference(this);
 }
 protected override void OnDestroy()
 {
     ModalActivityManager.ClearReference(this);
     base.OnDestroy();
 }
 protected override void OnResume()
 {
     base.OnResume();
     ModalActivityManager.RegisterCurrent(this);
 }