Deleting Project affecting the Merge Requests in the source project

Summary

Deleting Project affecting the Merge Requests in the source project

Steps to reproduce

1. Fork any one project
2. Do some changes and submit MR to the source branch
3. Delete the forked branch
4. Merge Requests will automatically get closed
5. Open the Merge Request that you have submitted to the source branch
6. You will get 500 error

Example MR

I deleted my forked project yoginth/gitlab-ce (Now I transferred this Project to my Bot Account deletece/gitlab-ce-old)

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28045 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28045

Output of checks

This bug happens on GitLab.com