private void CancelButton_Click(object sender, RoutedEventArgs e)
 {
     try
     {
         Client.CancelInvitation(SenderName);
     }
     catch (FaultException <UserFaultException> ex)
     {
         MessageBox.Show(ex.Detail.Message, "Error!", MessageBoxButton.OK, MessageBoxImage.Error);
     }
     this.Close();
 }