ActionView::Template::Error: undefined local variable or method `merge_request' for #<#<Class:0x0...

https://sentry.gitlap.com/gitlab/gitlabcom/issues/4280/

NameError: undefined local variable or method `merge_request' for #<#<Class:0x007f98eca4e3e0>:0x007f98eca44020>
  app/views/projects/merge_requests/invalid.html.haml:1:in `_app_views_projects_merge_requests_invalid_html_haml___2961432663548038672_70147390965860'
    - page_title "#{@merge_request.title} (#{merge_request.to_reference}", "Merge Requests"
  app/controllers/projects/merge_requests_controller.rb:361:in `invalid_mr'
    render 'invalid'
  app/controllers/projects/merge_requests_controller.rb:319:in `validates_merge_request'
    return invalid_mr unless @merge_request.source_project
  lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/metrics/transaction.rb:45:in `run'
    yield
...
(125 additional frame(s) were not displayed)

ActionView::Template::Error: undefined local variable or method `merge_request' for #<#<Class:0x007f98eca4e3e0>:0x007f98eca44020>
  app/views/projects/merge_requests/invalid.html.haml:1:in `_app_views_projects_merge_requests_invalid_html_haml___2961432663548038672_70147390965860'
    - page_title "#{@merge_request.title} (#{merge_request.to_reference}", "Merge Requests"
  app/controllers/projects/merge_requests_controller.rb:361:in `invalid_mr'
    render 'invalid'
  app/controllers/projects/merge_requests_controller.rb:319:in `validates_merge_request'
    return invalid_mr unless @merge_request.source_project
  lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/metrics/transaction.rb:45:in `run'
    yield
...
(125 additional frame(s) were not displayed)