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

https://sentry.gitlab.net/gitlab/gitlabcom/issues/616603/

NoMethodError: undefined method `xmlschema' for nil:NilClass
  /opt/gitlab/embedded/service/gitlab-rails/app/views/projects/tags/index.atom.builder:5:in `_app_views_projects_tags_index_atom_builder___127897005957125139_70154329020820'
    xml.updated @releases.first.updated_at.xmlschema if @releases.any?
  action_view/template.rb:159:in `block in render'
    view.send(method_name, locals, buffer, &block)
  active_support/notifications.rb:166:in `instrument'
    yield payload if block_given?
  action_view/template.rb:354:in `instrument'
    ActiveSupport::Notifications.instrument("!render_template.action_view".freeze, payload, &block)
  action_view/template.rb:157:in `render'
    instrument("!render_template".freeze) do
...
(163 additional frame(s) were not displayed)

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

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