Fix PageCache: async rendering of blocks can corrupt layout cache #8554

Author: adrian-martinez-interactiv4

app/code/Magento/PageCache/Controller/Block.php

@@ -68,7 +68,9 @@ protected function _getBlocks()
         $blocks = $this->jsonSerializer->unserialize($blocks);
         $handles = $this->base64jsonSerializer->unserialize($handles);
 
-        $handles[] = $this->additionalPageCacheHandle;
+        if(is_array($handles)){
+            $handles[] = $this->additionalPageCacheHandle;
+        }
 
         $this->_view->loadLayout($handles, true, true, false);
         $data = [];