NoMethodError: undefined method `unsubscribe' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1196373/

NoMethodError: undefined method `unsubscribe' for nil:NilClass
  sent_notifications_controller.rb:17:in `unsubscribe_and_redirect'
    noteable.unsubscribe(@sent_notification.recipient, @sent_notification.project)
  sent_notifications_controller.rb:10:in `unsubscribe'
    return unsubscribe_and_redirect if current_user || params[:force]
  action_controller/metal/basic_implicit_render.rb:6:in `send_action'
    super.tap { default_render unless performed? }
  abstract_controller/base.rb:194:in `process_action'
    send_action(method_name, *args)
  action_controller/metal/rendering.rb:30:in `process_action'
    super
...
(128 additional frame(s) were not displayed)

NoMethodError: undefined method `unsubscribe' for nil:NilClass