Ios iPad上共享按钮的应用程序崩溃

我已经创建了一个应用程序，你可以在其中分享你的分数。它在iPhone上运行得很好，但在iPad上它会使应用程序崩溃。我一直在AppDelegate中得到一个“线程1：信号SIGABRT”

class GameViewController: UIViewController, GameSceneDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()
        if let skView = self.view as? SKView {
            if skView.scene == nil{

                // create the scene

                let scene = GameScene(size: CGSize(width: 2048, height: 1536), delegate: self, gameState: .MainMenu)
                skView.showsFPS = false
                skView.showsNodeCount = false
                skView.ignoresSiblingOrder = true

                scene.scaleMode = .AspectFill

                skView.presentScene(scene)

            }
        }
        self.canDisplayBannerAds = true


    }
    override func prefersStatusBarHidden() -> Bool {
        return true
    }

    func screenshot() -> UIImage {


        UIGraphicsBeginImageContextWithOptions(view.bounds.size, false, 1.0)
        view.drawViewHierarchyInRect(view.bounds, afterScreenUpdates: true)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image


    }

    func shareString(string: String, url: NSURL, image: UIImage) {
        let vc = UIActivityViewController(activityItems: [string, url, image], applicationActivities: nil)
        presentViewController(vc, animated: true, completion: nil)
    }
}

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请将此代码与现有func共享字符串一起附加

if youractivityView.respondsToSelector("popoverPresentationController") {
        youractivityView.popoverPresentationController.sourceView = parentView
    }

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可能的副本，如果需要更多的帮助，然后让我知道