如何正确初始化类

以下代码在线路上产生错误 pickUp.spawnShield() 每当我尝试调用它时,都会从我的GameSecene类中删除。

var pickUp: Pickup! 为零。如果是这样,我如何正确初始化它?

class Pickup {

    var game: GameScene!

    func spawnShield() {

        let randomXStart = Random().random(min: game.gameArea.minX + game.player.size.width/2, max: game.gameArea.maxX - game.player.size.width/2)

        let randomXEnd = Random().random(min: game.gameArea.minX + game.player.size.width/2, max: game.gameArea.maxX - game.player.size.width/2)

        let startPoint = CGPoint(x: randomXStart, y: game.size.height * 1.1)
        let endPoint = CGPoint(x: randomXEnd, y: -game.size.height * 0.1)

        let shield = SKSpriteNode(imageNamed: "shieldDrop")
        shield.setScale(2)
        shield.position = startPoint
        shield.zPosition = 3

        game.addChild(shield)

        let moveShield = SKAction.move(to: endPoint, duration: TimeInterval(Helper().randomBetweenTwoNumbers(firstNumber: 3, secondNumber: 5)))

        shield.run(moveShield)
    }
}

然后我在课堂上这样称呼它:

class GameScene: SKScene, SKPhysicsContactDelegate {

    var pickUp: Pickup!

    override init(size: CGSize) {

        ....

        pickUp = Pickup() //something like this?

        super.init(size: size)
    }

    func startNewLevel() {

        pickUp.spawnShield()
    }
}