在Swift中扩展和泛化类SKSpriteNode时崩溃

Question

首先，我修改了文件GameScene，将一个新的简单类MySprite作为SKSpriteNode的子类添加，以显示一个红色矩形。代码运行良好：

import SpriteKit

class MySprite: SKSpriteNode {
    var scence: SKScene?

    init(theParent: SKScene) {
        super.init(texture: nil, color: UIColor.redColor(), size: CGSizeMake(300, 300))
        self.position = CGPointMake(500, 500)
        scence = theParent
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class GameScene: SKScene {
    override func didMoveToView(view: SKView) {    
        var mySprite = MySprite(theParent: self)
        self.addChild(mySprite)
    }
}

然后我将该类转换为通用类，如下所示：

import SpriteKit

class MyBase {
    var property: Int = 0
}

class MyGame: MyBase {
    var id: Int = 0
}

class MySprite<B: MyBase>: SKSpriteNode {
    var scence: SKScene?

    init(theParent: SKScene) {
        super.init(texture: nil, color: UIColor.redColor(), size: CGSizeMake(300, 300))
        self.position = CGPointMake(500, 500)
        scence = theParent // <- crash here EXC_BAD_ACCESS
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class GameScene: SKScene {
    override func didMoveToView(view: SKView) {
        var mySprite = MySprite<MyGame>(theParent: self)
        self.addChild(mySprite)
    }
}

编译好但在运行时遇到错误：在MySprite类的行scence = theParent中出现EXC_BAD_ACCESS。我已经检查过，MySprite的瞬间已经创建并且看起来很正常。

关于问题和解决方案的任何想法？非常感谢！