将item.picture添加到我的eBay产品列表中

Question

我想将我的Web应用程序中的新产品添加到我的沙盒eBay用户项目列表中。我在将图像添加到创建的产品中时遇到问题

错误：一行上的ExceptionSystem.NullReferenceException：

urlpic = item.PictureDetails.ExternalPictureURL.ToString();

static ItemType BuildItem(Product product)
        {
            ItemType item = new ItemType();

            // item title
            item.Title = product.ProdName;

            // item description
            item.Description = product.Description;
            eBayPictureService pictureService = new eBayPictureService(apiContext);
            pictureService.ApiContext.EPSServerUrl = @"https://api.sandbox.ebay.com/ws/api.dll";
            string urlpic = "";
            urlpic = item.PictureDetails.ExternalPictureURL.ToString();

            urlpic = product.ImageUrl;
        }