public function getUnregisteredDeviceTokens(array $devices_token)
    {
        /**
         * If there is any failure sending the notification
         */
        if ($this->feedback && isset($this->feedback->failure)) {
            $unRegisteredTokens = $devices_token;

            /**
             * Walk the array looking for any error.
             * If no error, unset it from all token list which will become the unregistered tokens array.
             */
            foreach ($this->feedback->results as $key => $message) {
                if (!isset($message->error)) {
                    unset($unRegisteredTokens[$key]);
                }
            }

            return $unRegisteredTokens;
        }

        return [];
    }