Error 500 viewing a merge request: undefined method `compare_in' for nil:NilClass

https://sentry.gitlap.com/gitlab/devgitlaborg/issues/286469/

NoMethodError: undefined method `compare_in' for nil:NilClass
  merge_request_diff.rb:185:in `diffs'
    if without_files? && comparison = diff_refs.compare_in(project)
  gitlab/metrics/instrumentation.rb:159:in `block in diffs'
    .measure { super }
  gitlab/metrics/method_call.rb:34:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:159:in `diffs'
    .measure { super }
  merge_request.rb:370:in `diffs'
    merge_request_diff.diffs(diff_options)
...
(184 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `compare_in' for nil:NilClass

ActionView::Template::Error: undefined method `compare_in' for nil:NilClass