private function processDefaultMenuItem(array $backendConfig)
    {
        $defaultMenuItem = $this->findDefaultMenuItem($backendConfig['design']['menu']);

        if ('empty' === $defaultMenuItem['type']) {
            throw new \RuntimeException(\sprintf('The "menu" configuration sets "%s" as the default item, which is not possible because its type is "empty" and it cannot redirect to a valid URL.', $defaultMenuItem['label']));
        }

        $backendConfig['default_menu_item'] = $defaultMenuItem;

        return $backendConfig;
    }