Can’t unwrap Optional.None

追踪这一点并不容易.是什么导致这个错误？

码：

import UIKit

class WelcomeViewController: UIViewController {
    let cornerRad:CGFloat = 10.0
    @IBOutlet var label:UILabel
    @IBOutlet var lvl1:UIButton
    @IBOutlet var lvl2:UIButton
    @IBOutlet var lvl3:UIButton
    init(nibName nibNameOrNil: String?,bundle nibBundleOrNil: NSBundle?) {
        super.init(nibName: nibNameOrNil,bundle: nibBundleOrNil)
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        lvl1.layer.cornerRadius = cornerRad
        lvl2.layer.cornerRadius = cornerRad
        lvl3.layer.cornerRadius = cornerRad
    }
}