Set subject, teacher, and room only when found (during exams import)

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

@@ -87,7 +87,7 @@ def import_exams(
                 lesson_id = int(el[0])
                 subject_id = int(el[1])
                 lesson = Lesson.objects.get(validity=validity_range, lesson_id_untis=lesson_id)
-                subject = subjects_ref[subject_id]
+                subject = subjects_ref[subject_id] if subject_id in subjects_ref else None
             first = False
             period = int(el[4])
             if not period:
@@ -96,8 +96,8 @@ def import_exams(
             period = time_periods_ref[weekday][period]
             teacher_id = int(el[5])
             room_id = int(el[6])
-            teacher = teachers_ref[teacher_id]
-            room = rooms_ref[room_id]
+            teacher = teachers_ref[teacher_id] if teacher_id in teachers_ref else None
+            room = rooms_ref[room_id] if room_id in rooms_ref else None
             exams.append((period, teacher, room))
 
         if not lesson or not subject or not exams: