From David Mudrak -

I am experiencing a regression while testing 3.5 upgrade of moodle.org which is likely related to this issue.

Our custom plugin there is sending a notification that does not have the courseid set. The courseid is not a required property of \core\message\message as per its inline docs. Yet \core\message\manager::send_message() seems to expect it be part of the $eventdata.

To clarify, it is \core\event\notification_sent which was introduced as a part of this issue, which throws exception when courseid is not set - see its validate_data() method.

 The thrown coding exception is

The 'courseid' value must be set in other.