private function assertMaintenanceMode()
    {
        /** @var ContainerHelper $helper */
        $helper = $this->getHelper('container');
        $app = $helper->getContainer();
        /** @var \Bolt\Config $config */
        $config = $app['config'];
        if ($config->get('general/maintenance_mode') !== true) {
            throw new RuntimeException('Site is not in maintenance mode!');
        }
    }