无法将空值插入到订单表中

我在向数据库中插入记录时遇到问题，错误说："无法将值 NULL 插入到'订单号'列，表'收据.dbo.订单';列不允许空值。插入失败。然后该语句已终止"，但是当我检查我的代码时，我似乎找不到任何错误。有人可以帮助我吗？提前感谢您：

这是我的代码集：

用于保存销售订单-

private void SaveSalesOrder(string status)
        {                
            int nOrder = 0;
            CloseConnection();  
            OpenConnection();
            trnOrder = cn.BeginTransaction();
            SqlCommand cmdInsert = new SqlCommand();
            try
            {
                cmdInsert.Connection = cn;
                cmdInsert.Transaction = trnOrder;
                cmdInsert.CommandType = CommandType.Text;
                cmdInsert.CommandText =
                    "INSERT INTO Orders " +
                    "(OrderDate, CustomerNo, CustomerName, CustomerAddress, PurchaseOrderNo, AgentName, Status) " +
                    "VALUES ('" +
                    dtpOrderDate.Value.Date.ToString() + "', '" +
                    txtCustomerNo.Text + "', '" +
                    txtCustomerName.Text + "', '" +
                    txtCustomerAddress.Text + "', '" +
                    txtPONo.Text + "', '" +
                    cboAgentName.Text + "', '" +
                    status + "'); " +
                    "SELECT TOP 1 OrderNo FROM Orders " +
                    "ORDER BY OrderNo DESC;";
                nOrder = Convert.ToInt16(cmdInsert.ExecuteScalar().ToString());
                for (int nRow = 0;
                    nRow <= dsDetail.Tables["OrderDetails"].Rows.Count - 1;
                    nRow++) 
                {                      
                    double dQuantity = Convert.ToDouble(dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["Quantity"].ToString());
                    string strUnit = dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["Unit"].ToString();
                    int nProductNo = Convert.ToInt16(dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["ProductNo"].ToString());
                    string strProductName = dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["ProductName"].ToString();
                    string strProductSize = dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["ProductSize"].ToString();
                    string strPackagingInside = dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["PackagingInside"].ToString();
                    double dSellingDiscount = Convert.ToDouble(dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["SellingDiscount"].ToString());
                    double dSellingPrice = Convert.ToDouble(dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["SellingPrice"].ToString());
                    double nAmount = Convert.ToDouble(dsDetail.Tables["OrderDetails"].
                        Rows[nRow]["Amount"].ToString());
                    SqlCommand cmdInsertDetail = new SqlCommand();
                    cmdInsertDetail.Connection = cn;
                    cmdInsertDetail.Transaction = trnOrder;
                    cmdInsertDetail.CommandType = CommandType.Text;
                    cmdInsertDetail.CommandText =
                        "INSERT INTO OrderDetails " +
                        "(OrderNo, PackagingOutside, Quantity, Unit, ProductNo, ProductName, " +
                        "ProductSize, PackagingInside, SellingDiscount, SellingPrice, Amount) " +
                        "VALUES ('" +
                        nOrder + "', '" +
                        dPackagingOutside + "', '" +
                        dQuantity + "', '" +
                        strUnit + "', '" +
                        nProductNo + "', '" +
                        strProductName + "', '" +
                        strProductSize + "', '" +
                        strPackagingInside + "', '" +
                        dSellingDiscount + "', '" +
                        dSellingPrice + "', '" +
                        nAmount + "')";
                    cmdInsertDetail.ExecuteNonQuery();
                }
                trnOrder.Commit();
                if (status == "OK")
                {
                    MessageBox.Show("Transaction has been saved!", "Success",
                    MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                else
                {
                    MessageBox.Show("Transaction has been voided!", "Void Transaction",
                    MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
            }
            catch (SqlException ex)
            {
                trnOrder.Rollback(); 
                MessageBox.Show(ex.Message);
            }
            finally
            {
                cn.Close();
            }
        } 
private void btnSave_Click(object sender, EventArgs e)
    {
        if (txtCustomerNo.Text == "")
        {
            MessageBox.Show("Please select a customer first.", "Empty", MessageBoxButtons.OK, MessageBoxIcon.Error);
            btnSearchCustomer.Focus();
            return;
        }
        if (grdDetails.Rows.Count < 1)
        {
            MessageBox.Show("Please select a product first.", "Empty",
                MessageBoxButtons.OK, MessageBoxIcon.Error);
            btnProductSearch.Focus();
            return;
        }
        SaveSalesOrder("OK");
        groupCustomer(false);
        groupProduct(false);
        CSalesInv.EnableDisable(this, false);
        CloseConnection();
        InitializeOrder();
        lblTotal.Text = "";
    }