Hittest why does program shut down

Question

I am trying to create a easy game where the user is supposed to hit on a rectangle in timely maner. I tried with hittestresult which made it possible to add points and create a new rectangle when it gets clicked on. But every time you click on any other space of the canvas the program shuts down. why does this happen. I am just a beginner so excuse my bad programming skills. Here is my method containing the issue:

    private void spielfeld_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {

        // Retrieve the coordinate of the mouse position.
        Point pt = e.GetPosition((UIElement)sender);

        // Perform the hit test against a given portion of the visual object tree.
        HitTestResult treffer = VisualTreeHelper.HitTest(spielfeld, pt);

        if (treffer != null)
        {
            string name = ((Shape)(treffer.VisualHit)).Name;

            if (name == "fangbutton")
            {
                punkte++;

                punktAnzeige.Content = punkte;

                catchme.Entfernen(spielfeld);

                catchme.Anzeige(spielfeld);
                

            }


        }