UINavigationControllerのpushViewControllerで画面遷移できない

前提・実現したいこと

ボタンをタップした際に
UINavigationController の pushViewController を使って画面遷移したいです。

発生している問題・エラーメッセージ

swift
1func toSignupForm() {
2        let navigator = DefaultSignupFormNavigator(navigationController: UINavigationController())
3        let viewController = SignupFormViewController(navigator: navigator)
4        navigationController?.pushViewController(viewController, animated: true)
5    }

でエラーはでず、画面遷移しません。

該当のソースコード

swift
1import UIKit
2
3@UIApplicationMain
4class AppDelegate: UIResponder, UIApplicationDelegate {
5    var window: UIWindow?
6    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
7        FirebaseApp.configure()
8        window = UIWindow(frame: UIScreen.main.bounds)
9        let navigationController = UINavigationController()
10        let navigator = DefaultWelcomeNavigator(navigationController: navigationController)
11        window?.rootViewController = WelcomeViewController(navigator: navigator)
12        window?.makeKeyAndVisible()
13        return true
14    }
15}

swift
1import UIKit
2
3class WelcomeViewController: UIViewController {
4    let navigator: WelcomeNavigator
5    
6    init(navigator: WelcomeNavigator) {
7        self.navigator = navigator
8        super.init(nibName: nil, bundle: nil)
9    }
10    
11    required init?(coder: NSCoder) {
12        fatalError("init(coder:) has not been implemented")
13    }
14    
15    override func viewDidLoad() {
16        super.viewDidLoad()
17        title = "fanew"
18        view.backgroundColor = UIColor.white
19        view.addSubview(buttonStackView)
20    }
21    
22    private lazy var buttonStackView: UIStackView = {
23        let stackView = UIStackView(arrangedSubviews: [signupButton, signinButton])
24        stackView.frame = CGRect(x: 0, y: 300, width: self.view.frame.width, height: self.view.frame.height / 4)
25        stackView.axis = .vertical
26        stackView.alignment = .center
27        stackView.distribution = .equalSpacing
28        stackView.spacing = 3
29        return stackView
30    }()
31    
32    private lazy var signupButton: UIButton = {
33        let button = UIButton()
34        button.setTitle("新規登録", for: .normal)
35        button.setTitleColor(.black, for: .normal)
36        button.addTarget(self, action: #selector(pushSignupButton), for: .touchUpInside)
37        return button
38    }()
39
40    private lazy var signinButton: UIButton = {
41        let button = UIButton()
42        button.setTitle("ログイン", for: .normal)
43        button.setTitleColor(.black, for: .normal)
44        button.addTarget(self, action: #selector(pushSigninButton), for: .touchUpInside)
45        return button
46    }()
47    
48    @objc func pushSignupButton(sender: UIButton){
49        navigator.toSignupForm()
50    }
51    
52    @objc func pushSigninButton(sender: UIButton){
53        navigator.toSigninForm()
54    }
55    
56}

import UIKit

protocol WelcomeNavigator {
    func toSignupForm()
    func toSigninForm()
}

final class DefaultWelcomeNavigator: WelcomeNavigator {
    private var navigationController: UINavigationController?
    init(navigationController: UINavigationController?) {
        self.navigationController = navigationController
    }
    
    func toSignupForm() {
        let navigator = DefaultSignupFormNavigator(navigationController: UINavigationController())
        let viewController = SignupFormViewController(navigator: navigator)
        navigationController?.pushViewController(viewController, animated: true)
    }
    
    func toSigninForm() {
        let viewController = SigninFormViewController()
        navigationController?.pushViewController(viewController, animated: true)
    }
}