public function get_plugin_by_type($subtype, $type) {
        $shortsubtype = substr($subtype, strlen('assign'));
        $name = $shortsubtype . 'plugins';
        if ($name != 'feedbackplugins' && $name != 'submissionplugins') {
            return null;
        }
        $pluginlist = $this->$name;
        foreach ($pluginlist as $plugin) {
            if ($plugin->get_type() == $type) {
                return $plugin;
            }
        }
        return null;
    }