Opening merge request returns 500 error with - NoMethodError: undefined method `+' for nil:NilClass (NoMethodError)

Sentry Issue: GITLABCOM-AGT7

NoMethodError: undefined method `+' for nil:NilClass (NoMethodError)

    @commits_count = @merge_request.commits_count + @merge_request.context_commits_count
                                                  ^
  app/controllers/projects/merge_requests_controller.rb:471:in `render_html_page'
    @commits_count = @merge_request.commits_count + @merge_request.context_commits_count
  app/controllers/projects/merge_requests_controller.rb:425:in `block (2 levels) in show_merge_request'
    render_html_page
  app/controllers/projects/merge_requests_controller.rb:420:in `show_merge_request'
    respond_to do |format|
  app/controllers/projects/merge_requests_controller.rb:102:in `show'
    show_merge_request
  app/controllers/application_controller.rb:537:in `block in allow_gitaly_ref_name_caching'
    yield
...
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