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

https://sentry.gitlab.net/gitlab/gitlabcom/issues/961972/

NoMethodError: undefined method `to_ability_name' for nil:NilClass
  notifications_helper.rb:117:in `show_unsubscribe_title?'
    can?(current_user, "read_#{noteable.to_ability_name}".to_sym, noteable)
  /opt/gitlab/embedded/service/gitlab-rails/app/views/sent_notifications/unsubscribe.html.haml:3:in `_app_views_sent_notifications_unsubscribe_html_haml__307532839197046829_70303393183540'
    - noteable_text = show_unsubscribe_title?(noteable) ? %(#{noteable.title} (#{noteable.to_reference})) : %(#{noteable.to_reference})
  action_view/template.rb:159:in `block in render'
    view.send(method_name, locals, buffer, &block)
  active_support/notifications.rb:170:in `instrument'
    yield payload if block_given?
  action_view/template.rb:354:in `instrument_render_template'
    ActiveSupport::Notifications.instrument("!render_template.action_view".freeze, instrument_payload, &block)
...
(152 additional frame(s) were not displayed)

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

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