エラー発生 C#

System.NullReferenceException: 'オブジェクト参照がオブジェクト インスタンスに設定されていません。'

ArenaDeBatalha.GameLogic.GameObject.Sprite.get が null を返しました。

public abstract class GameObject
    {

        #region Game Object Properties
        public Bitmap Sprite { get; set; }

        public bool Active { get; set; }

        public int Speed { get; set; }

        public int Left { get; set; }

        public int Top { get; set; }

        public int Width { get { return this.Sprite.Width; } }

        public int Height { get { return this.Sprite.Height; } }

        public Size Bounds { get; set; }

        public Rectangle Square { get; set; }

        public Stream Sound { get; set; }

        public Graphics Screen { get; set; }

        private SoundPlayer soundPlayer { get; set; }
        #endregion

        #region Game Object Methods

        public abstract Bitmap GetSprite();

        public GameObject(Size bounds, Graphics screen)
        {
            this.Bounds = bounds;
            this.Screen = screen;
            this.Active = true;
            this.soundPlayer = new SoundPlayer();
        }

        public virtual void InitializeGraphics()
        {
            this.Sprite = this.GetSprite();
            this.Square = new Rectangle(this.Left, this.Top, this.Width, this.Height);
        }

        public virtual void UpDateObject()
        {
            this.Square = new Rectangle(this.Left, this.Top, this.Width, this.Height);
            this.Screen.DrawImage(this.Sprite, this.Square);
        }