Changes to the Messaging system in 3.6 have cause Scheduler email notifications to stop going out. 

WHAT YOU EXPECTED

1. I click "Send Invitation" on a Scheduler activity
2. I send a message to multiple users
3. I see "Message sent to X users"
4. Emails successfully send

WHAT ACTUALLY HAPPENS

1. I click "Send Invitation" on a Scheduler activity
2. I send a message to multiple users
3. I see "Message sent to X users"
4. No emails show up in users' inboxes

There is a trap in the 3.6 code where no messages get sent.

Specifically, line 95 here  https://github.com/moodle/moodle/commit/52f9cf205e45f591ac637e15608ba5cb543d6184#diff-d17043b0cdd06fbbb352434c9c1f8edcR95

part of 

https://tracker.moodle.org/browse/MDL-63724