Fix PHPStan errors

src/Wallabag/CoreBundle/Controller/AbstractController.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Wallabag\CoreBundle\Controller;
+
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController as BaseAbstractController;
+use Wallabag\UserBundle\Entity\User;
+
+abstract class AbstractController extends BaseAbstractController
+{
+    /**
+     * @return User|null
+     */
+    protected function getUser()
+    {
+        $user = parent::getUser();
+        \assert(null === $user || $user instanceof User);
+
+        return $user;
+    }
+}