错误会告诉您出了什么问题。
您正尝试在viewDidLoad中显示视图控制器，但该视图虽然已加载，但不在任何层次结构中。请尝试将代码放入viewDidAppear方法中，该方法在视图出现在屏幕上后调用，并且在视图层次结构中。

Swift 4更新我认为这可以帮助您解决您的问题：

override func viewDidLoad() {
    super.viewDidLoad()

    verificationCode.delegate = self

    let alert = UIAlertController(title: "Oops!", message:"This feature isn't available right now", preferredStyle: .alert)
    let delete = UIAlertAction(title: "OK", style: .default) { (_) in }
    let cancelAction = UIAlertAction(title: "Cancel", style: .cancel) { (_) in }
    alert.addAction(cancelAction)
    alert.addAction(delete)

    alert.popoverPresentationController?.sourceView = sender as UIView
    UIApplication.shared.keyWindow?.rootViewController?.present(alert, animated: true, completion: nil)

    if (!Util.isConnectedToNetwork()) {
        self.isConnected = false
    }
}

将代码从viewDidLoad移动到viewDidAppear

override func viewDidAppear(animated: Bool) {
    // Delegates
    verificationCode.delegate = self

    let alert = UIAlertController(title: "Oops!", message:"This feature isn't available right now", preferredStyle: .Alert)
    let action = UIAlertAction(title: "OK", style: .Default) { _ in }
    alert.addAction(action)
    self.presentViewController(alert, animated: true) {}

    if (!Util.isConnectedToNetwork()) {
        self.isConnected = false
    }

}