Discussion view posts from all periods does not work

[geoffreykwan]

1. Author a Discussion component and enable "Students can discuss with students from all periods, not just their own"
2. Sign in as a student and go to the Discussion component
3. Submit a post and after that it will then try to retrieve all the other student posts
4. The request to retrieve the other posts will fail. It makes a request to the endpoint below and one of the parameters is null.

http://localhost/api/classmate/discussion/student-work/6/null/node12/ijypxikxi1