public final function get_config($key) {
        if (!in_array($key, $this->get_allowed_config())) {
            $a = (object)array('property' => $key, 'class' => get_class($this));
            throw new portfolio_export_exception($this->get('exporter'), 'invalidconfigproperty', 'portfolio', null, $a);
        }
        if (isset($this->config->{$key})) {
            return $this->config->{$key};
        }
        return null;
    }