Course Completion Report Can't Be Viewed if Required Module is Disabled

Problem

If an activity is included in a course's completion criteria, and then the activity type is disabled at the site level, the Course Completion Report throws and error:

Exception - Call to a member function get_formatted_name() on null

Expected behavior

A site admin should be able disable activities and either 1) silently skip the disabled activity in completion requirements or 2) warn the user that some required activities are not available.

Steps to reproduce

1. In a course with at least one student
2. Add a chat Activity
3. Select it under condition: Activity Completion in the Course Completion criteria
4. In site administration, under Plugins > Activity Modules > Manage Activities,
5. Disable the chat activity
6. Return to the course and verify the activity is not present
7. Check that it also cannot be deselected (it does not appear) in the Completion Criteria settings
8. View Reports > Course Completion and get an error ('Call to a member function get_formatted_name() on null')