MR from forked project throws exception when forked project is removed: ActionView::Template::Error: 5:GetRepoPath: not a git repository

https://sentry.gitlab.net/gitlab/gitlabcom/issues/854684/

GRPC::NotFound: 5:GetRepoPath: not a git repository '/var/opt/gitlab/git-data/repositories/@hashed/(REDACTED).git'
  grpc/generic/active_call.rb:31:in `check_status'
    fail GRPC::BadStatus.new_status_exception(
  grpc/generic/active_call.rb:181:in `attach_status_results_and_complete_call'
    recv_status_batch_result.check_status
  grpc/generic/active_call.rb:377:in `request_response'
    attach_status_results_and_complete_call(batch_result)
  grpc/generic/client_stub.rb:178:in `block in request_response'
    c.request_response(req, metadata: metadata)
  grpc/generic/interceptors.rb:170:in `intercept!'
    return yield if @interceptors.none?
...
(221 additional frame(s) were not displayed)

Gitlab::Git::Repository::NoRepository: 5:GetRepoPath: not a git repository '/var/opt/gitlab/git-data/repositories/@hashed/(REDACTED).git'
  gitlab/git/wraps_gitaly_errors.rb:9:in `rescue in wrapped_gitaly_errors'
    raise Gitlab::Git::Repository::NoRepository.new(e)
  gitlab/git/wraps_gitaly_errors.rb:6:in `wrapped_gitaly_errors'
    def wrapped_gitaly_errors(&block)
  gitlab/metrics/instrumentation.rb:161:in `block in wrapped_gitaly_errors'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `wrapped_gitaly_errors'
    .measure { super }
...
(212 additional frame(s) were not displayed)

ActionView::Template::Error: 5:GetRepoPath: not a git repository '/var/opt/gitlab/git-data/repositories/@hashed/(REDACTED).git'

ActionView::Template::Error: 5:GetRepoPath: not a git repository '/var/opt/gitlab/git-data/repositories/@hashed/(REDACTED).git'

check affected MR here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24690