如何在没有情节提要文件的情况下将另一个viewController推送到屏幕上？我已经尝试了下面的代码，但是什么也没有发生

import UIKit

class SecondViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .blue
        title = "Second View Controller"
    }
}

import UIKit

class FirstViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        let button = UIButton(type: .system)
        button.setTitle("Go to Second VC", for: .normal)
        button.addTarget(self, action: #selector(pushSecondViewController), for: .touchUpInside)
        view.addSubview(button)
        button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])
    }

    @objc func pushSecondViewController() {
        let secondViewController = SecondViewController()
        navigationController?.pushViewController(secondViewController, animated: true)
    }
}

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    window = UIWindow(frame: UIScreen.main.bounds)
    let firstViewController = FirstViewController()
    window?.rootViewController = UINavigationController(rootViewController: firstViewController)
    window?.makeKeyAndVisible()
    return true
}