public function validation($data, $files) {
        $errors = parent::validation($data, $files);

        list($instance, $plugin, $context, $type) = $this->_customdata;

        $pluginerrors = $plugin->edit_instance_validation($data, $files, $instance, $context);

        $errors = array_merge($errors, $pluginerrors);

        return $errors;
    }