Discovered by dag while testing MDLQA-19266

I've managed to replicate the problem on the QA site, here's the steps:

1. As the admin go to Site administration / Users / Privacy and policies / Privacy settings.
2. Ensure that "Automatic data deletion request approval" and "Create automatic data deletion requests" are enabled.
3. As a student in the course and participate in a few activities, including adding a new discussion topic in the forum and replying to a forum post.
4. As an administrator and go to Site administration / Users / Accounts / Browse list of users and delete the "Sam student" user.
5. The following exception will be displayed: