为什么UIButton选择器是随机崩溃而不是直接崩溃？

Question

我知道我应该像arbitraryAction()而不是arbitraryAction(_ complition: (() -> Void)?)那样实现选择器。只看下面的代码。如果您点击Button，代码将随机崩溃，这使我发疯。我只想知道为什么崩溃是随机的！

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        let button = UIButton.init(frame: CGRect.init(x: 100, y: 100, width: 100, height: 100))
        button.backgroundColor = .red
        button.addTarget(self, action: #selector(self.arbitraryAction(_:)), for: .touchUpInside)
        view.addSubview(button)
    }

    @objc func arbitraryAction(_ complition: (() -> Void)?) {
    }

}