Hi Team

This issue seems to be related to MDL-73309 which occurs due to calendar subscriptions are kept when their assigned groups are deleted.

With the calendar setting "Admin see all" activated, this causes an exception.

Steps:

1. Activate "calendar_adminseesall" setting
2. Create a course and add a group
3. Navigate to the Calendar
4. Import a calendar subscription for that group of that course   
5. Delete the group in step 1
6. Allow the task "\core\task\calendar_cron_task" to process the calendar subscription
7. View Calendar with "Admin see all" setting active

Observations:

• the calendar subscription with orphan group exists with a non-existing group link
• the cron task creates events for those calendar subscriptions
• Exception (with "calendar_adminseesall" ) 

Exception - property_exists(): Argument #1 ($object_or_class) must be of type object|string, bool given