LaravelFCM\Response\DownstreamResponse::needToAddError PHP Method

    private function needToAddError($index, $result)
    {
        if (array_key_exists(self::ERROR, $result)) {
            if ($this->tokens[$index]) {
                $this->tokensWithError[$this->tokens[$index]] = $result[self::ERROR];
            }
        }
    }