swift – 类型UIViewController的值在Xcode 7更新后没有成员topV

override func viewDidLoad() {
    super.viewDidLoad()
    self.navigationItem.leftBarButtonItem = self.editButtonItem()
    let addButton = UIBarButtonItem(barButtonSystemItem: .Search,target: self,action: "addButtonPressed:")
    self.navigationItem.rightBarButtonItem = addButton

    if let split = self.splitViewController {
        let controllers = split.viewControllers
        self.detailViewController =
            controllers[controllers.count-1].topViewController as? ### Error heres
        DetailViewController
    }

    model = Model(delegate: self) // create the Model
    model.synchronize() // tell model to sync its data
    self.navigationController?.toolbarHidden = false;
    self.navigationItem.backBarButtonItem = UIBarButtonItem(title:"Home",style:.Plain,target:nil,action:nil)
}