thank you very much for your comments! I post an answer for completeness: yes guys you are right, that's not the Binding that was responsible for the crash. I have a function which binds a TextBox.Text to SelectedItem of ListView as follows:

    (listView.SelectedItem as Element).Patient.SurName = (sender as TextBox).Text;

.. and I did not check if SelectedItem is not null. After checking this all worked fine.