nslayoutconstraint - iOS でプログラムで制約を設定すると、ボタンが非表示になります

Question

私はこのコードを持っています。ブロックのコメントを外すと、ボタンがなくなり、コメントが表示されます。コメント解除時

    var fab:UIButton = UIButton(type: .custom)

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = UIColor.white
        self.title = "Maintain My Vehicle"
        let vehicleInfoController = VehicleInfoController()
        vehicleInfoController.tabBarItem = UITabBarItem(tabBarSystemItem: .favorites, tag: 0)
        let vehicleLogsController = VehicleLogsController()
        vehicleLogsController.tabBarItem = UITabBarItem(tabBarSystemItem: .bookmarks, tag: 2)
        let viewControllerList = [ vehicleInfoController, vehicleLogsController ]
        viewControllers = viewControllerList.map { UINavigationController(rootViewController: $0) }
        floatingButton()
        let centerLabel = UILabel()

        // Don't forget this!
        centerLabel.translatesAutoresizingMaskIntoConstraints = false

        centerLabel.text = "Perfectly centered!"
        view.addSubview(centerLabel)

        NSLayoutConstraint.activate([
            centerLabel.centerXAnchor.constraint(
                equalTo: view.centerXAnchor, constant: 0),
            centerLabel.centerYAnchor.constraint(
                equalTo: view.centerYAnchor, constant: 0)
            ])
        /*
        fab.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            fab.centerXAnchor.constraint(
                equalTo: view.centerXAnchor, constant: 0),
            fab.centerYAnchor.constraint(
                equalTo: view.bottomAnchor, constant: 0)
            ])
 */
    }


func floatingButton(){
    fab = UIButton(type: .custom)
    fab.frame = CGRect(x: 150, y: 600, width: 75, height: 75)
    fab.setTitle("+", for: .normal)
    fab.backgroundColor = #colorLiteral(red: 0.1764705926, green: 0.4980392158, blue: 0.7568627596, alpha: 1)
    fab.clipsToBounds = true
    fab.layer.cornerRadius = 50
    fab.layer.borderColor = #colorLiteral(red: 1, green: 1, blue: 1, alpha: 1)
    fab.layer.borderWidth = 3.0
    fab.layer.masksToBounds = true
    fab.layer.zPosition = 1
    fab.addTarget(self, action:#selector(self.buttonClicked), for: .touchUpInside)
    view.addSubview(fab)
}

コメントされた時

中央のラベルが問題ない理由がわかりませんが、ボタンがめちゃくちゃです。