无法在SpriteKit中播放动画

Question

我有一个简单的游戏场景，想在游戏结束后重设游戏。我将.sks用于GameScene。view.presentScene确实运行良好，但是以下代码无法正常工作。

我该如何解决？

if let view = self.view {

    if let scene = SKScene(fileNamed: "GameScene") {
        scene.scaleMode = .aspectFill
        view.ignoresSiblingOrder = true
        view.showsFPS = true
        view.showsNodeCount = true

        let transition = SKTransition.fade(withDuration: 1.0)
        view.presentScene(scene, transition: transition)
                        view.presentScene(scene, transition: SKTransition.crossFade(withDuration: 1.0)) //didn't work

        //view.presentScene(scene) // work

    }

    view.ignoresSiblingOrder = true
    view.showsFPS = true
    view.showsNodeCount = true
}

更新
如果我使用过渡，则发生错误Fatal error: Unexpectedly found nil while unwrapping an Optional value。

    override func didMove(to view: SKView) {

        self.backgroundColor = UIColor.blue

        ball = self.childNode(withName: "Ball") as? SKSpriteNode
        paddle = self.childNode(withName: "Paddle") as? SKSpriteNode
        scoreLabel = self.childNode(withName: "Score") as? SKLabelNode

        let urlPath = Bundle.main.url(forResource: "success", withExtension: "mp3")
        do {
            audioPlayer = try AVAudioPlayer(contentsOf: urlPath!)
            audioPlayer.prepareToPlay()
        } catch {
            print("Error!")
        }

        ball.physicsBody?.applyImpulse(CGVector(dx: 50, dy: 50)) // error because ball is nil

        let border = SKPhysicsBody(edgeLoopFrom: (view.scene?.frame)!)
        border.friction = 1
        self.physicsBody = border

        self.physicsWorld.contactDelegate = self

    }