Add null check for database result

Author: Jan-Schuppik

library/Notifications/Daemon/Daemon.php

@@ -263,7 +263,7 @@ protected function processNotifications(): void
         /** @var IncidentHistory $notification */
         $notificationsToProcess = [];
         foreach ($notifications as $notification) {
-            if (isset($connections[$notification->contact_id])) {
+            if ($notification->contact_id !== null && isset($connections[$notification->contact_id])) {
                 ObjectsRendererHook::register($notification->incident->object);
                 $notificationsToProcess[] = $notification;