Сheck if user is authenticated in RBAC UserGroupRule.

docs/guide/security-authorization.md

@@ -416,6 +416,7 @@ You can create set up the RBAC data as follows,
 ```php
 namespace app\rbac;
 
+use Yii;
 use yii\rbac\Rule;
 
 /**
@@ -427,15 +428,16 @@ class UserGroupRule extends Rule
 
     public function execute($user, $item, $params)
     {
-        $group = \Yii::$app->user->identity->group;
+        if (!Yii::$app->user->isGuest) {
+            $group = Yii::$app->user->identity->group;
             if ($item->name === 'admin') {
                 return $group == 1;
             } elseif ($item->name === 'author') {
                 return $group == 1 || $group == 2;
-        } else {
-            return false;
             }
         }
+        return false;
+    }
 }
 
 $rule = new \app\rbac\UserGroupRule;