protected function renderBackendForm($instance, array $fields)
    {
        /*
         * Prepare all names & ids
         */
        $fieldIds = [];
        $fieldNames = [];
        foreach ($fields as $f) {
            $fieldIds = array_merge($fieldIds, [
                $f => $this->get_field_id($f),
            ]);
            $fieldNames = array_merge($fieldNames, [
                $f => $this->get_field_name($f),
            ]);
        }
        $instance = array_merge($instance, [
            'fieldId' => $fieldIds,
            'fieldName' => $fieldNames,
        ]);

        return $this->mustacheRender->render(
            $this->getTemplateName(static::$backFileName),
            [
                'instance' => $instance,
            ]
        );
    }