1. Visit a course.
2. Add the calendar block if it doesn't exist.
3. Click on the name of the month.
4. Create a new event with default values.
5. Notice you get the error 'Coding error detected, it must be fixed by a programmer: Undefined property requested'.

The reason is the groupid is set to null so if (!isset($this->properties->{$key})) { fails. You can fix this by editing the event mapper to set the groupid to 0 instead of null but that seems like a hack.