This issue has been raised previously under MDL-40081 but was unable to be reproduced and was discussed by people experiencing it here https://moodle.org/mod/forum/discuss.php?d=387180 

Steps to reproduce.

Normal operation 

• Go to course with Course Completion enabled. 
• Go to course completion screen 
• Mark activities that need to be completed with ALL options required. 
• Add a course to 'Condition: Completion of other courses'
• Submit / complete activities in the course
• Complete the required course
• Run the cron
• Go to course completion progress block - report is okay and visible

Breaking behaviour

• Course Completion criteria will be locked after submission.  
• Unlock course completion criteria
• Change from ALL options to ANY options. 
• Add / remove an existing tick or option
• Save

 OR

• Delete required course
• Navigate to the course with Course Completion enabled
• You will se an error: "Notice: Trying to get property 'id' of non-object in /var/www/site/completion/criteria/completion_criteria_course.php on line 138"

When course completion options come up again it includes a completion criteria that other courses be completed with no other courses filled in - even though it was not selected by the user above. 

If you go to course completion progress block again - gives error.

Can't find data record in database table course.

More information about this error

—

Debug info: SELECT id,category FROM {course} WHERE id IS NULL
[array (
)]
Error code: invalidrecord

—

Stack trace:

• line 1546 of /lib/dml/moodle_database.php: dml_missing_record_exception thrown
• line 1522 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
• line 6818 of /lib/accesslib.php: call to moodle_database->get_record()
• line 138 of /completion/criteria/completion_criteria_course.php: call to context_course::instance()
• line 409 of /report/completion/index.php: call to completion_criteria_course->get_title_detailed()

After looking around I found some records in the database that had null modules. 

You can identify them with

SELECT * FROM prefix_course_completion_criteria where module is null; 

By deleting these null records I was able to access the Course Completion reports that I was unable to access before. I also found that this was somehow linked to criteriatype 8 or criteria type 7 in the database. 

It may be unrelated but I also found a lot of duplicated values in prefix_course_completions. I literally had hundreds of duplicated values for the same courses that was affected by the predix_course_completion_criteria. 

I was able to reproduce the problem and it seems that it breaks not because of the null value on the criteriatype=7 records but rather because of the single criteriatype 8 record that gets added by https://github.com/moodle/moodle/blob/master/completion/criteria/completion_criteria_course.php at line 85 onwards. 

Hope this helps someone to identify and fix this problem which seems to be have been around for a while.