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

func toSignupForm() {
        let navigator = DefaultSignupFormNavigator(navigationController: UINavigationController())
        let viewController = SignupFormViewController(navigator: navigator)
        navigationController?.pushViewController(viewController, animated: true)
    }

でエラーはでず、画面遷移しません。

該当のソースコード

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        FirebaseApp.configure()
        window = UIWindow(frame: UIScreen.main.bounds)
        let navigationController = UINavigationController()
        let navigator = DefaultWelcomeNavigator(navigationController: navigationController)
        window?.rootViewController = WelcomeViewController(navigator: navigator)
        window?.makeKeyAndVisible()
        return true
    }
}

import UIKit

class WelcomeViewController: UIViewController {
    let navigator: WelcomeNavigator

    init(navigator: WelcomeNavigator) {
        self.navigator = navigator
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        title = "fanew"
        view.backgroundColor = UIColor.white
        view.addSubview(buttonStackView)
    }

    private lazy var buttonStackView: UIStackView = {
        let stackView = UIStackView(arrangedSubviews: [signupButton, signinButton])
        stackView.frame = CGRect(x: 0, y: 300, width: self.view.frame.width, height: self.view.frame.height / 4)
        stackView.axis = .vertical
        stackView.alignment = .center
        stackView.distribution = .equalSpacing
        stackView.spacing = 3
        return stackView
    }()

    private lazy var signupButton: UIButton = {
        let button = UIButton()
        button.setTitle("新規登録", for: .normal)
        button.setTitleColor(.black, for: .normal)
        button.addTarget(self, action: #selector(pushSignupButton), for: .touchUpInside)
        return button
    }()

    private lazy var signinButton: UIButton = {
        let button = UIButton()
        button.setTitle("ログイン", for: .normal)
        button.setTitleColor(.black, for: .normal)
        button.addTarget(self, action: #selector(pushSigninButton), for: .touchUpInside)
        return button
    }()

    @objc func pushSignupButton(sender: UIButton){
        navigator.toSignupForm()
    }

    @objc func pushSigninButton(sender: UIButton){
        navigator.toSigninForm()
    }

}

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)
    }
}

補足情報（FW/ツールのバージョンなど）

Swift5、Xcode11.3.1