dsLinq.Count() 引发了“System.NullReferenceException”类型的异常

DataTable dt = PurchaseArriveInfoBus.GetPurchaseArriveInfo(companyCD, txtArriveNo, txtTitle, txtProvider);
        XElement dsXML = ConvertDataTableToXML(dt);
        //linq排序
        var dsLinq =
            (order == "ascending") ?
            (from x in dsXML.Descendants("Data")
             orderby x.Element(orderBy).Value ascending
             select new DataSourceModel()
             {
                 ID = x.Element("ID").Value,
                 ArriveNo = x.Element("ArriveNo").Value,
                 Title = x.Element("Title").Value,
                 ProviderID = x.Element("ProviderName").Value,
                 Purchaser = x.Element("Purchaser").Value,
                 ProductNo = x.Element("ProductNo").Value,
                 ProductName = x.Element("ProductName").Value,
                 ColorName = x.Element("ColorName").Value,
                 
                 TotalPrice = x.Element("TotalPrice").Value,
                 ProductCount = x.Element("ProductCount").Value,
                 InCount = x.Element("InCount").Value,
                 DetailID = x.Element("DetailID").Value,
                 UnitName = x.Element("UnitName").Value,
                 JiBenCount = x.Element("JiBenCount").Value,
                 UnitPrice = IsDisplayPrice ? x.Element("UnitPrice").Value : "0.000000"

             })
                      :
            (from x in dsXML.Descendants("Data")
             orderby x.Element(orderBy).Value descending
             select new DataSourceModel()
             {
                 ID = x.Element("ID").Value,
                 ArriveNo = x.Element("ArriveNo").Value,
                 Title = x.Element("Title").Value,
                 ProviderID = x.Element("ProviderName").Value,
                 Purchaser = x.Element("Purchaser").Value,
                 ProductNo = x.Element("ProductNo").Value,
                 ProductName = x.Element("ProductName").Value,
                 ColorName = x.Element("ColorName").Value,
                 
                 TotalPrice = x.Element("TotalPrice").Value,
                 ProductCount = x.Element("ProductCount").Value,
                 InCount = x.Element("InCount").Value,
                 DetailID = x.Element("DetailID").Value,
                 UnitName = x.Element("UnitName").Value,
                 JiBenCount = x.Element("JiBenCount").Value,
                 UnitPrice = IsDisplayPrice ? x.Element("UnitPrice").Value : "0.000000"
             });
        int totalCount = dsLinq.Count();