在尝试将导航控制器呈现为rootViewController时，不在View Hierachy中查看

@IBAction func logOut(sender: AnyObject) {
    PFUser.logOutInBackground()


    let MainView: UIViewController = self.presentingViewController!

    self.dismissViewControllerAnimated(true, completion: {let storyboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
    let vc: UIViewController = storyboard.instantiateViewControllerWithIdentifier("ViewController") as UIViewController

    let navigationController = UINavigationController(rootViewController: vc)

        MainView.presentViewController(navigationController, animated: true, completion: nil)})
}
@IBAction func logOut(sender: AnyObject) {