public override void Execute(object parameter)
 {
     if (_view == null)
     {
         _view = new AdListView(new AdListViewModel());
         _view.Closed += view_Closed;
         _view.Show();
     }
     else
     {
         _view.Activate();
     }
 }
 private void view_Closed(object sender, EventArgs e)
 {
     _view = null;
 }