Deleting branch with opened MR

Summary

The UI for viewing an open MR of a deleted branch makes it look as though there is an issue with GitLab.

Steps to reproduce

1. Create a branch
2. Do something on that branch (ex/ add a file ..etc)
3. Open a Merge Request for that change
4. Go to the Repository tab
5. Click on the Branches tab
6. Delete your branch
7. Go back to the opened MR

Expected behavior

I think there should be some kind of message that says that the branch was deleted instead of a flash message saying that Something went wrong on our end

Actual behavior