Use messenger method not property (not set)

Problem/Motivation

The messenger property of DefaultController is not set - use the messenger() method of the parent class' MessengerTrait.

Steps to reproduce

Flush the facebook cache for a node, the error message "Call to a member function addMessage() on null in Drupal\facebook_flush_cache\Controller\DefaultController->execute()" is shown.

Proposed resolution

Attached patch uses the messenger() method.