private void Cb2_SelectionChanged(object sender, SelectionChangedEventArgs e)
 {
     using (Repos rep = new Repos())
     {
         TempList2 = rep.Leerlings(TempList[Cb2.SelectedIndex]);
         foreach (Leerling l in TempList2)
         {
             Leerlingen.Add(l.Naam);
         }
     }
     Cb3.ItemsSource = Leerlingen;
 }