private void Delete_Click(object sender, RoutedEventArgs e)
 {
     if (listBox.SelectedItem != null)
     {
         _contentManager.DeleteTopic((listBox.SelectedItem as Topic).ID);
         _topics.Remove(listBox.SelectedItem as Topic);
         textBox.Text      = "";
         _saved            = false;
         Edit.IsEnabled    = false;
         Delete.IsEnabled  = false;
         Confirm.IsEnabled = true;
         if (_topics.Count == 0)
         {
             _empty = true;
             EditFiles.IsEnabled   = false;
             EditLessons.IsEnabled = false;
         }
     }
 }