private void BtnNew_Click(object sender, RoutedEventArgs e)
        {
            AddCdWindow addCdWindow = new AddCdWindow(null);

            addCdWindow.ShowDialog();

            Repository repo = Repository.GetInstance();

            _cds = repo.GetAllCds();
            listBoxCds.ItemsSource = _cds;
        }
        private void BtnEdit_Click(object sender, RoutedEventArgs e)
        {
            Cd selectedCd = listBoxCds.SelectedItem as Cd;

            if (selectedCd == null)
            {
                MessageBox.Show("Sie müssen eine CD auswählen!");
                return;
            }
            AddCdWindow addCdWindow = new AddCdWindow(selectedCd);

            addCdWindow.ShowDialog();
            Repository repo = Repository.GetInstance();

            _cds = repo.GetAllCds();

            listBoxCds.ItemsSource = _cds;
        }