To Reproduce
============
- add event on calendar
- edit the event and remove the user from participants list and add another one.
- save the record -> traceback
Problem
=======
we try to read `hasReadAccess` from `this.followedThread` that is `null` in this case.
Solution
========
check if `this.followedThread` is `null` or not
opw-3126545
closesodoo/odoo#109721
X-original-commit: f9a23af63bb5fe882f0d42f06ef1406910836115
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>