Fix error with uninstantiated resource model

Author: schmengler

app/code/Magento/Catalog/Model/Category.php

@@ -303,7 +303,17 @@ protected function _construct()
      */
     protected function getCustomAttributesCodes()
     {
-        return $this->_resource->getCustomAttributesCodes();
+        return $this->_getResource()->getCustomAttributesCodes();
+    }
+
+    /**
+     * @throws \Magento\Framework\Exception\LocalizedException
+     * @return \Magento\Catalog\Model\ResourceModel\Category
+     * @deprecated because resource models should be used directly
+     */
+    protected function _getResource()
+    {
+        return parent::_getResource();
     }
 
     /**