为什么ViewController后退按钮在Xcode上不起作用？

import UIKit

class FirstViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        self.navigationItem.title = "First VC"
        
        let nextButton = UIBarButtonItem(title: "Next", style: .plain, target: self, action: #selector(nextAction))
        self.navigationItem.rightBarButtonItem = nextButton
    }
    
    @objc func nextAction() {
        let secondViewController = SecondViewController()
        navigationController?.pushViewController(secondViewController, animated: true)
    }
}

class SecondViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        self.navigationItem.title = "Second VC"
        
        let backButton = UIBarButtonItem(title: "Back", style: .plain, target: self, action: #selector(backAction))
        self.navigationItem.leftBarButtonItem = backButton
    }
    
    @objc func backAction() {
        navigationController?.popViewController(animated: true)
    }
}

let firstViewController = FirstViewController()
let navigationController = UINavigationController(rootViewController: firstViewController)
window?.rootViewController = navigationController