private void BtnNew_Click(object sender, RoutedEventArgs e)
        {
            AddAndEditWindow addAndEditWindow = new AddAndEditWindow(null);

            addAndEditWindow.ShowDialog();
            RefreshList();
        }
        private void BtnEdit_Click(object sender, RoutedEventArgs e)
        {
            Member member = listBoxMembers.SelectedItem as Member;

            if (member == null)
            {
                MessageBox.Show("Member auswählen");
                return;
            }

            AddAndEditWindow addAndEditWindow = new AddAndEditWindow(member);

            addAndEditWindow.ShowDialog();
            RefreshList();
        }