Adding pages domain with any validation error result in 500

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1560636/?referrer=gitlab_plugin

NoMethodError: undefined method `pages_domain' for #<PagesDomain:0x00007f45ea6e9330>
Did you mean?  pages_virtual_domain
  active_model/attribute_methods.rb:431:in `method_missing'
    match ? attribute_missing(match, *args, &block) : super
  /opt/gitlab/embedded/service/gitlab-rails/app/views/projects/pages_domains/_certificate.html.haml:39:in `_app_views_projects_pages_domains__certificate_html_haml___2165917135387853340_69969227150780'
    = @domain.pages_domain.subject || _('missing')
  action_view/base.rb:274:in `_run'
    send(method, locals, buffer, &block)
  action_view/template.rb:185:in `block in render'
    view._run(method_name, self, locals, buffer, &block)
  active_support/notifications.rb:182:in `instrument'
    yield payload if block_given?
...
(207 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `pages_domain' for #<PagesDomain:0x00007f45ea6e9330>
Did you mean?  pages_virtual_domain

ActionView::Template::Error: undefined method `pages_domain' for #<PagesDomain:0x00007f45ea6e9330>
Did you mean?  pages_virtual_domain