奇怪的ASP表单NullReferenceException：对象引用未设置为对象的实例

Question

我在“创建”视图中有一个表单。当我尝试显示某个字段时，我得到NullReferenceException: Object reference not set to an instance of an object.。如果我注释掉ItemId的输入控件，一切都很好。标签显示，但显然不是控件。

品种：

public class Variety
{
    [Key]
    public int ItemId { get; set; }

    public int SpeciesId { get; set; }
    public virtual Species species { get; set; }

    [DataType(DataType.Currency, ErrorMessage = "Please ender a currency value")]
    [DisplayFormat(DataFormatString = "{0:C}", ApplyFormatInEditMode = true)]
    public double Price { get; set; }

    [Required(ErrorMessage = "You must enter a variety.")]
    [Display(Name = "Variety")]
    public string VarietyName { get; set; }

}

InventoryItem：

public class InventoryItem
{
    [Key]
    [Display(Name = "Inventory Id")]
    public int InventoryId { get; set; }

    [Required(ErrorMessage = "Item Id Required")]
    [Display(Name = "Item Id")]
    public int ItemId { get; set; }
    public virtual Variety variety { get; set; }

    [Required(ErrorMessage = "Quantity Required")]
    public int Quantity { get; set; }
}

查看：

@using (Html.BeginForm()) 
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <h4>New Inventory Item</h4>
        <hr />
        <div class="form-group">
           <label for="ItemId", class = "control-label col-md-2">Item Id</label>
            <div class="col-md-10">
                <input type="text" value="@Model.ItemId" id="ItemId" readonly />
            </div>
        </div>
    ....
    ....