如何在最新的swift中编写NSAlert？

func alertUser() {
        let alert = NSAlert()
        alert.messageText = "message 1"
        alert.informativeText = "info1"
        alert.informativeText = "info2"
        alert.addButton(withTitle: "NO")
        alert.addButton(withTitle: "YES")
        alert.beginSheetModal(for: self.view.window!) { (returnCode: NSModalResponse) -> Void in
            print ("returnCode: ", returnCode)
        }

但是,当我在行alert.beginSheetModal上打开一个Optional值消息时,我发现了可怕的意外发现的nil

请告诉我我做错了什么.

谢谢

override func viewDidAppear() {
    super.viewDidAppear()

    alertUser()
}