fix sending notification when unsetting a referrer from a period

When the user removes the referrer, a notification was send to a `null`
addressee, which causes an unhandled exception.

src/Bundle/ChillPersonBundle/AccompanyingPeriod/Events/UserRefEventSubscriber.php

@@ -60,6 +60,7 @@ class UserRefEventSubscriber implements EventSubscriberInterface
     {
         if ($period->hasPreviousUser()
             && $period->getUser() !== $this->security->getUser()
+            && null !== $period->getUser()
             && $period->getStep() !== AccompanyingPeriod::STEP_DRAFT
         ) {
             $this->generateNotificationToUser($period);