private function initializeConfig()
    {
        if (empty($this->config['url'])) {
            $this->config['url'] = static::$url;
        }

        $this->config['url'] = rtrim($this->config['url'], '/');

        if (filter_var($this->config['url'], FILTER_VALIDATE_URL) === false) {
            throw new InvalidConfigException('You must provide a valid URL in the config.');
        }
    }