private class func getTabBarController() -> UITabBarController? {
        let appDelegate = UIApplication.shared.delegate as? AppDelegate
        guard let rootViewController = appDelegate?.window?.rootViewController?.childViewControllers.first as? UITabBarController else { return nil }
        return rootViewController
    }

    private class func navigationControllerFrom(tabBarController: UITabBarController) -> UINavigationController {
        var navigationController: UINavigationController
        if tabBarController.selectedIndex <= tabBarController.childViewControllers.count {
            navigationController = tabBarController.childViewControllers[tabBarController.selectedIndex] as! UINavigationController
        } else {
            navigationController = tabBarController.childViewControllers[0] as! UINavigationController
        }
        return navigationController
    }