C＃XNA错误框“NullReferenceException未处理”，“对象引用未设置为对象的实例”。

Question

完全相同的代码行使用下面几行，它工作得很好。我的老师无法理解。我在Visual Basic C＃中使用XNA。当我运行我的游戏错误消息“NullReferenceException未处理”，“对象引用未设置为对象的实例。”显示。

表示错误的行：

Size = new Rectangle(0, 0, (int)(mBallTexture.Width * Scale),(int)(mBallTexture.Height * Scale));

我的代码：

 class RandomBall
        {
            //The asset name for the Sprite's Texture
            public string AssetName;

            //The size of the Sprite
            public Rectangle Size;

            //Used to size the Sprite up or down from the original image
            public float mScale = 1.0f;

            //The current position of the Sprite
            public Vector2 Position = new Vector2(500, 0);

            //The texture object used when drawing the sprite
            private Texture2D mBallTexture ;




            //When the scale is modified throught he property, the Size of the 
            //sprite is recalculated with the new scale applied.
            public float Scale
            {
                get {return mScale;}
                set
                {
                    mScale = value;
                    //Recalculate the Size of the Sprite with the new scale


    //THIS CODE LINE IS THE ONE CAUSING TROUBLE
    Size = new Rectangle(0, 0, (int)(mBallTexture.Width * Scale), (int)(mBallTexture.Height * Scale));

                    ////Size = new Rectangle(0, 0, 50, 50);
                }
            }


            //Load the texture for the sprite using the Content Pipeline
            public void LoadContent(ContentManager theContentManager, string theAssetName)
            {
                mBallTexture = theContentManager.Load<Texture2D>(theAssetName);
                AssetName = theAssetName;
                Size = new Rectangle(0, 0, (int)(mBallTexture.Width * Scale), (int)(mBallTexture.Height * Scale));
            }