如果退出View Controller，则关闭SVProgressHUD

Question

我有一个视图控制器，在启动时显示SVProgressHUD，因为数据是从服务器获取的。如果在获取完成之前移回上一个视图控制器，我将如何关闭SVProgressHUD？

我试过这个：

override func viewWillDisappear(animated : Bool) {
     super.viewWillDisappear(animated)

        if (self.isMovingFromParentViewController()){
            print("before dismiss")
            SVProgressHUD.dismiss()
            print("after dismiss")
        }
}

我收到错误：致命错误：在解包可选值时意外发现nil

以下是视图控制器中导航回的一些代码：

override func viewWillAppear(animated: Bool) {
        self.navigationItem.hidesBackButton = true
    }

    override func viewDidLoad() {
        searchRestaurants.delegate = self
        loadRestaurants()
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        //navigation bar title edit
        self.navigationController!.navigationBar.titleTextAttributes = [ NSFontAttributeName: UIFont(name: "Chalkboard SE", size: 17)!]

        //fetch list of restaurant names from database
        //set table cells to values inside array
    }

    override func viewDidAppear(animated: Bool) {
        //SVProgressHUD.dismiss()
    }

    override func viewWillDisappear(animated : Bool) {
        super.viewWillDisappear(animated)

        if (self.isMovingFromParentViewController()){
            let qualityOfServiceClass = QOS_CLASS_BACKGROUND
            let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
            dispatch_async(backgroundQueue, {
                print("This is run on the background queue")
                SVProgressHUD.dismiss()
            })
        }
    }