Hi, I wanted to make an app that have one big button that change title after its tapped. Also, I'm making it in code, not in storyboards. But when I tap button, code crashes. Do you know what I'm doing wrong?

Here's my code:

var mainButton: UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()

        let mainButton = UIButton(type: .system)
        mainButton.setTitle("Hello, tap here!", for: .normal)
        mainButton.setTitleColor(.white, for: .normal)
        mainButton.titleLabel?.font = UIFont.systemFont(ofSize: 40)
        mainButton.translatesAutoresizingMaskIntoConstraints = false
        mainButton.addTarget(self, action: #selector(mainButtonTapped), for: .touchUpInside)
        view.addSubview(mainButton)

        NSLayoutConstraint.activate([
        mainButton.centerXAnchor.constraint(equalTo: view.layoutMarginsGuide.centerXAnchor),
        mainButton.centerYAnchor.constraint(equalTo: view.layoutMarginsGuide.centerYAnchor, constant: -20),
        mainButton.widthAnchor.constraint(equalTo: view.layoutMarginsGuide.widthAnchor),
        mainButton.heightAnchor.constraint(equalTo: view.layoutMarginsGuide.heightAnchor, constant: -100)
        ])
    }

    @objc func mainButtonTapped() {
        mainButton.setTitle("Tapped", for: .normal)
    }

Thank you so much, now it looks so simple :D