存储过程出错：无法启用约束。一行或多行包含违反非空，唯一或外键约束的值

Question

我在项目中的DataSet（“SDMDAL.xsd”）中使用DataAdapter（“SDM_Tran_GenerateInvoice”）。

下面是DataAdapter的结构以及其中的存储过程名称：

下面是我使用的表结构：

我在类文件中将此DataAdapter称为 SDM.InvoiceBLL.cs ：

using SDMDALTableAdapters;

public class SDM_Invoice
{
    private SDM_Tran_GenerateInvoiceTableAdapter _GenerateInvoiceTableAdapter = null;
    protected SDM_Tran_GenerateInvoiceTableAdapter Adapter
    {
        get
        {
            if (_GenerateInvoiceTableAdapter == null)
                _GenerateInvoiceTableAdapter = new SDM_Tran_GenerateInvoiceTableAdapter();

            return _GenerateInvoiceTableAdapter;
        }
    }

    #region GET
    public SDMDAL.SDM_Tran_GenerateInvoiceDataTable SelectInvoice(string _SPID)
    {
        return Adapter.SelectInvoice(_SPID);         }
    public SDMDAL.SDM_Tran_GenerateInvoiceDataTable GetInvoiceBillingBySPID(string _SPID)
    {
        return Adapter.GetInvoiceBillingBySPID(_SPID); //getting error for this Stored Procedure

    }
    public SDMDAL.SDM_Tran_GenerateInvoiceDataTable GetInvoiceID()
    {
        return Adapter.GetInvoiceID();
    }
    public SDMDAL.SDM_Tran_GenerateInvoiceDataTable GetInvoiceNumber()
    {
        return Adapter.GetInvoiceNumber();
    }
    #endregion
}

然后在 Default.aspx.cs 页面中调用类文件的 SelectInvoice（）函数，以在RadGrid中显示记录：

protected void rgInvoice_NeedDataSource(object sender, GridNeedDataSourceEventArgs e)
{
        rgInvoice.DataSource = SDM.Invoice.GetInvoiceBillingBySPID(lblId.Text);
}

以下是 GetInvoiceBillingBySPID（_SPID）存储过程

ALTER PROCEDURE [dbo].[SDM_Select_BillingBySPID_Invoice]
    @SPID as nvarchar(50)
    AS
BEGIN

    SET NOCOUNT ON; 
SELECT 
--B.ID,
B.BillingID, 
DCIDescription, --
(SELECT BUName FROM dbo.SDM_Master_BU WITH (NOLOCK) WHERE BUID=AfoBUID) as BUName, --
(SELECT BUfoStatusID FROM dbo.SDM_Master_BU WITH (NOLOCK) WHERE BUID=AfoBUID) as BUfoStatusID --
FROM SDM_Tran_Billing B WITH (NOLOCK),
dbo.SDM_Tran_DCI WITH (NOLOCK), 
dbo.SDM_Tran_Allocation WITH (NOLOCK)
WHERE B.BfoAllocationID=AllocationID and AfoDCIID=DCIID and DCIfoSPID=@SPID and B.BfoStatusID=1

END

但是当我运行 Default.aspx 页面时，每次我在行return Adapter.GetInvoiceBillingBySPID(_SPID);

下面出现以下错误

以下是错误快照 - 源文件中显示的位置代码：

请注意，我是DataAdapter和Asp.Net（C＃）的新手。
请让我知道我在代码中犯了什么错误这个错误是什么意思？请以简单的方式回复，以便我能理解。
提前致谢