private void ThisWindowClosed(object sender, CancelEventArgs e)
        {
            DataWriter.Create("data.xml");
            DataWriter writer = new DataWriter("data.xml");

            foreach (Fach f in _viewModel1.Other)
            {
                writer.AddFach(f);
            }

            writer.AddStundenplan("DU", _viewModel1.CurrentUser.Stundenplan);
        }