Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/lernid/widgets/Slide.py", line 144, in _session_changed
    if session.slides:
  File "/usr/lib/python2.6/dist-packages/lernid/Sessions.py", line 72, in slides
    return self._slides
AttributeError: 'Session' object has no attribute '_slides'