Merge request cannot be viewed when source project was deleted after it was merged
Example: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6503
Sentry (internal): https://sentry.gitlap.com/gitlab/gitlabcom/issues/11709/
Example: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6503
Sentry (internal): https://sentry.gitlap.com/gitlab/gitlabcom/issues/11709/