Error accessing MR: undefined method `namespace' for nil:NilClass

Accessing https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/417 currently results in an Error 500. Most likely because source branch no longer exists?

https://sentry.gitlap.com/gitlab/gitlabcom/issues/11709/

NoMethodError: undefined method `namespace' for nil:NilClass
  app/helpers/merge_requests_helper.rb:113:in `source_branch_with_namespace'
    branch = link_to(merge_request.source_branch, namespace_project_commits_path(merge_request.source_project.namespace, merge_request.source_project, merge_request.source_branch))
  app/views/projects/merge_requests/_show.html.haml:35:in `_app_views_projects_merge_requests__show_html_haml__2153484404782715815_69955203268500'
    %span.label-branch= source_branch_with_namespace(@merge_request)
  app/views/projects/merge_requests/show.html.haml:1:in `_app_views_projects_merge_requests_show_html_haml___706992753412067225_69955203200640'
    = render "show"
  lib/gitlab/request_profiler/middleware.rb:15:in `call'
    @app.call(env)
  lib/gitlab/middleware/rails_queue_duration.rb:20:in `call'
    @app.call(env)
...
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ActionView::Template::Error: undefined method `namespace' for nil:NilClass