Fix sync of lessons with empty course

aleksis/apps/lesrooster/models.py

@@ -339,7 +339,8 @@ class Lesson(TeacherPropertiesMixin, RoomPropertiesMixin, ExtensibleModel):
 
         lesson_event.save()
 
-        lesson_event.groups.set(self.course.groups.all())
+        if self.course:
+            lesson_event.groups.set(self.course.groups.all())
         lesson_event.teachers.set(self.teachers.all())
         lesson_event.rooms.set(self.rooms.all())