Fix PKF-369: check null when get access token in getUserInfo function

Author: Phan Van Thanh

fproject/authclient/OAuth2.php

@@ -198,8 +198,8 @@ public function verifyAndDecodeToken($token, $checkRevoked=true)
     public function getUserInfo($accessToken=null, $cacheDuration=-1)
     {
         /** @var string $cacheKey */
-        if($accessToken == null)
-            $accessToken = $this->getAccessToken()->token;
+        if($accessToken == null && !empty($at = $this->getAccessToken()))
+            $accessToken = $at->token;
 
         $userInfo = null;