Error when closing MR for a deleted fork

When a fork is deleted the existing MR looks like this. reported here https://gitlab.com/gitlab-org/gitlab-ce/issues/4007

When trying to close the existing MR the following error is displayed.

/cc @dzaporozhets @DouweM

Added ~13921 memory usage ~12980 reproduced on GitLab.com labels

Removed memory usage label

Milestone changed to 8.7

Reassigned to @felipe_artur

mentioned in merge request !3664 (merged)

Status changed to closed by commit 8ea6c6d8

mentioned in commit 8ea6c6d8

Mentioned in commit pfjason/gitlab-ce@8ea6c6d8