如何从推送的控制器获取RootViewController？

import UIKit

var rootViewController: UIViewController?

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // 创建窗口
    let window = UIWindow(frame: UIScreen.main.bounds)
    
    // 创建RootViewController
    let rootViewController = UIViewController()
    rootViewController.view.backgroundColor = .white
    
    // 设置窗口的RootViewController
    window.rootViewController = rootViewController
    
    // 显示窗口
    window.makeKeyAndVisible()
    
    // 存储RootViewController
    self.rootViewController = rootViewController
    
    return true
}

func userNotificationCenter(_ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse, withCompletionHandler completionHandler: @escaping () -> Void) {
    // 获取RootViewController
    guard let rootViewController = self.rootViewController else {
        return
    }
    
    // 在这里处理推送通知的响应
    // ...
    
    // 完成处理
    completionHandler()
}