500 Error while accessing "Pull requests" (BB-7759)

Hello!

I've just got 500 error while trying to view my "Pull Requests" tab.

A problem seems to be reproduced in the following way:

1. Create user
2. Fork some public repository (let it be "A") into "B"
3. Make some changes in "B"
4. Create a "Pull Request" with the changes you made
5. Start "Pull Request" review with the administrator / admin grop of the "A" repository
6. Delete user created at step 1.

After that you have polling "Pull Request" in your repository "A", but you cannot see it because of a 500 error.

Debugging hash: b12a95a8e37d4e62b0b82b092f8193ec$a2fa515e48c169ba316d5696bb5c5ede

Thank you!