Notice: Trying to get property of non-object in /master/mod/assign/feedback/comments/locallib.php on line 137

Noticed this error while testing for MDL-33537.

Steps to reproduce:

1. Delete mod_assign and mod_assignment modules and install them again (it will be easier doing this, is to make both modules use the same grading instance itemid, this way will be 1)
2. Enable the assignment 2.2
3. Create a course and assign a student
4. Create a new assignment instance with rubrics as a grading method
5. Create a rubric for the new assignment with 1 criteria and 4 levels with values 1,2,3,4
6. Go to the new assignment; grade the student, add a "AAAA" text on the right box of the criteria and save changes

The above notice will be displayed.