Code review comment for lp:~justas.sadzevicius/schooltool/timetables-section-time-boundaries

Five tests in schooltool.timetable.tests.test_model now fail with

Traceback (most recent call last):
  ...
  File "/home/menesis/src/schooltool/trunk/build/schooltool-timetables/src/schooltool/timetable/model.py", line 130, in createCalendar
    if not first <= date <= last:
TypeError: can't compare datetime.date to NoneType