#65 LessonViewer bug when opening up an unlisted lessonfile

When one tries to open up another lesson (file), other
than the ones listed in the LessonViewer dialog and
clicks the "Cancel" button, instead of choosing a file
and pressing the "Ok" button, several windows after one
another pop up with error messages.

To reproduce:
1. Open up LessonViewer, enter "guest" as loginname and
click "Sign in" button
2. press the yellow 'folder button' right next to the
"Ok" button.
3. Now press the "Cancel" button in the filechooser dialog.
--> Several windows pop up with error messages

Jack, we need to fix this immediately!
Set priority to 8.