private void btn_DoneAddSubject_Click(object sender, RoutedEventArgs e)
        {
            bool erfolgreich = true;

            if (edit)
            {
                SubjectManager.EditSubject(subject.id, tbx_Name_AddSubject.Text, tbx_Teacher_AddSubject.Text, SubjectColorManager.GetList().Find(x => x.name == (gvw_ColorPicker_AddSubject.SelectedItem as SubjectColor).name));
            }
            else
            {
                erfolgreich = SubjectManager.AddSubject(tbx_Name_AddSubject.Text, tbx_Teacher_AddSubject.Text, (SubjectColor)gvw_ColorPicker_AddSubject.SelectedItem);
            }
            if (erfolgreich == true)
            {
                Frame.GoBack();
            }
        }