NoMethodError: undefined method `commit_status' for nil:NilClass

https://sentry.gitlap.com/gitlab/gitlabcom/issues/2634/

NoMethodError: undefined method `commit_status' for nil:NilClass
  app/controllers/projects/merge_requests_controller.rb:237:in `ci_status'
    status = ci_service.commit_status(merge_request.last_commit.sha, merge_request.source_branch)
  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
  lib/gitlab/metrics/rack_middleware.rb:17:in `call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/middleware/go.rb:16:in `call'
    @app.call(env)
...
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