Error 500 when viewing a diff

https://sentry.gitlap.com/gitlab/gitlabcom/issues/41608/

NoMethodError: undefined method `new' for nil:NilClass
  lib/gitlab/diff/file.rb:213:in `simple_viewer'
    @simple_viewer ||= simple_viewer_class.new(self)
  lib/gitlab/metrics/instrumentation.rb:156:in `block in simple_viewer'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  lib/gitlab/metrics/method_call.rb:23:in `measure'
    retval = yield
  lib/gitlab/metrics/instrumentation.rb:156:in `simple_viewer'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  app/views/projects/diffs/_content.html.haml:2:in `_app_views_projects_diffs__content_html_haml___3759556550658918483_70065597748160'
    = render 'projects/diffs/viewer', viewer: diff_file.rich_viewer || diff_file.simple_viewer
...
(185 additional frame(s) were not displayed)

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

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