guess_school_id migration does not work properly coming from Untis 2.1.3

  File "/usr/local/lib/python3.9/dist-packages/django/db/migrations/operations/special.py", line 190, in database_forwards
    self.code(from_state.apps, schema_editor)
  File "/usr/local/lib/python3.9/dist-packages/aleksis/apps/untis/migrations/0003_guess_school_id.py", line 16, in guess_school_id
    school_id = vr.school_id_untis
AttributeError: 'NoneType' object has no attribute 'school_id_untis'