Skip extra lessons without a subject

(cherry picked from commit e4060943)

CHANGELOG.rst

@@ -6,6 +6,19 @@ All notable changes to this project will be documented in this file.
 The format is based on `Keep a Changelog`_,
 and this project adheres to `Semantic Versioning`_.
 
+Unreleased
+----------
+
+Changed
+~~~~~~~
+
+* Wrap all imports in complete revisions to make it possible to undo them completely and to track changes correctly.
+
+Fixed
+~~~~~
+
+* Skip extra lessons without a subject.
+
 `2.0rc2`_ - 2021-07-30
 ----------------------
 

aleksis/apps/untis/util/mysql/importers/substitutions.py

@@ -180,6 +180,10 @@ def import_substitutions(
                 if not teacher_new and not teacher_old:
                     teachers = []
 
+                if not subject:
+                    logger.warning("  Skip because missing subject")
+                    continue
+
                 (extra_lesson, created,) = (
                     chronos_models.ExtraLesson.objects.select_related(None)
                     .prefetch_related(None)