c# NullReferenceException未处理-对象引用未设置为对象的实例

我试图在c#中使用Windows窗体创建一个应用程序，但是我一直得到一个错误陈述:

NullReferenceException是未处理的-对象引用未设置为对象的实例

它指向有这个代码的行:

carBootSaleList.AddCarBootSale(newCarBootSale);

我在表单界面上的方法:

  CarBootSaleList carBootSaleList;
  public void AddCarBootSale()
    {
        AddNewCarBootSale addForm = new AddNewCarBootSale();
        if (addForm.ShowDialog() == DialogResult.OK)
        {
            CarBootSale newCarBootSale = addForm.GetCarBootSaleData();
            carBootSaleList.AddCarBootSale(newCarBootSale);
            txtCarBootSale.Clear();
            txtCarBootSale.Text = newCarBootSale.Display();
        }
    }

CarBootSaleList类:

public class CarBootSaleList : IDisplay
{
    private List<CarBootSale> carbootsales;
    public CarBootSaleList()
    {
        carbootsales = new List<CarBootSale>();
    }
    public bool AddCarBootSale(CarBootSale carbootsale)
    {
        bool success = true;
        foreach (CarBootSale cbs in carbootsales)
        {
            if (cbs.ID == carbootsale.ID)
            {
                success = false;
            }
        }
        if (success)
        {
            carbootsales.Add(carbootsale);
        }
        return success;
    }
    public int GetListSize()
    {
        return carbootsales.Count();
    }
    //public CarBootSale FindCarBootSale(string cbsID)
    //{
    //    CarBootSale carbootsale = null;
    //    foreach (CarBootSale cbs in carbootsale)
    //    {
    //        if (cbs.ID == cbsID)
    //        {
    //            carbootsale = cbs;
    //        }
    //    }
    //    return carbootsale;
    //}
    public List<CarBootSale> ReturnList()
    {
        return carbootsales;
    }
    public string Display()
    {
        string msg = "";
        foreach (CarBootSale cbs in carbootsales)
        {
            msg += String.Format("{0}  {1}", cbs.ID, cbs.Location, cbs.Date);
            msg += Environment.NewLine;
        }
        return msg;
    }
}

CarBootSale类:

public class CarBootSale : IDisplay
{
    public string ID { get; set; }
    public string Date { get; set; }
    public string Location { get; set; }
    public double PitchCost { get; set; }
    public int Capacity { get; set; }
    public string Charity { get; set; }
    public string CharityName { get; set; }
    public string Catering { get; set; }
    public CarBootSale(string id, string date, string location, double pitchcost, int capacity, string charity, string charityname, string catering)
    {
        ID = id;
        Date = date;
        Location = location;
        PitchCost = pitchcost;
        Capacity = capacity;
        Charity = charity;
        CharityName = charityname;
        Catering = catering;
    }
    public CarBootSale() { }
    public override string ToString()
    {
        return String.Format("{0}  {1}", ID, Date, Location);
    }
    public string Display()
    {
        string msg;
        string CR = Environment.NewLine;
        msg = String.Format("ID: {0} {1}", ID, CR);
        msg += String.Format(" Date: {0} {1}", Date, CR);
        msg += String.Format(" Location: {0} {1}", Location, CR);
        msg += String.Format(" Pitch Cost: {0} {1}", PitchCost, CR);
        msg += String.Format(" Capacity: {0} {1}", Capacity, CR);
        msg += String.Format(" Charity: {0} {1}", Charity, CR);
        msg += String.Format(" CharityName: {0} {1}", CharityName, CR);
        msg += String.Format(" Catering: {0} {1}", Catering, CR);
        return msg;
    }
}

CarBootSaleList carBootSaleList;

CarBootSaleList carBootSaleList = new CarBootSaleList();

CarBootSaleList carBootSaleList = new CarBootSaleList();

CarBootSaleList carBootSaleList = new carBootSaleList();

 carBootSaleList = new CarBootSaleList();