private void BtnNew_Click(object sender, RoutedEventArgs e)
        {
            NewOrEditWindow newOrEditWindow = new NewOrEditWindow(null);

            newOrEditWindow.ShowDialog();

            PopulateListView();
        }
        private void BtnEdit_Click(object sender, RoutedEventArgs e)
        {
            if (listViewMembers.SelectedItem == null)
            {
                MessageBox.Show("Wählen Sie ein Mitglied zum bearbeiten aus!");
                return;
            }

            NewOrEditWindow newOrEditWindow = new NewOrEditWindow(listViewMembers.SelectedItem as Member);

            newOrEditWindow.ShowDialog();

            PopulateListView();
        }