private void Button_AddReservation(object sender, RoutedEventArgs e)
        {
            Window AddReservation = new ReservationHinzufügen(null);

            AddReservation.Show();
            this.Close();
        }
        private void Button_ModifyReservation(object sender, RoutedEventArgs e)
        {
            var    viewModel      = (ReservationViewModel)DataContext;
            Window AddReservation = new ReservationHinzufügen(viewModel.SelectedReservation);

            AddReservation.Show();
            this.Close();
        }