gridview1嵌套gridview2，在gridview2中使用dropdownlist问题。

Question

我在gridview2的GridView2_RowDataBound事件中，如下代码：
        //为DropDownList预算科目绑定值
        if (((DropDownList)e.Row.FindControl("addbodstitle")) != null)
        {
          
            DropDownList addbodstitle = (DropDownList)e.Row.FindControl("addbodstitle");

            addbodstitle.Items.Clear();
            //科目类别list修改更新使用
            SqlConnection modelsc = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["DinfoConnectionString"].ToString());
            string modelsql = "SELECT DISTINCT bsparentcode FROM Dinfo_BudgetSubject  WHERE bscntitle='" + ((DataRowView)GridView1.FooterRow.DataItem)["bocnclass"].ToString() + "   ORDER BY bscntitle";
            SqlDataAdapter msda = new SqlDataAdapter(modelsql, modelsc);
            modelsc.Open();
            DataSet modeldt = new DataSet();
            msda.Fill(modeldt, "MyTable");

            addbodstitle.AppendDataBoundItems = true;
            addbodstitle.Items.Add(new ListItem("---请选择---", "0"));
            addbodstitle.DataSource = modeldt.Tables["MyTable"].DefaultView;
            addbodstitle.DataTextField = "bscntitle";
            //DropDownList1.DataValueField = "MC001";
            addbodstitle.DataBind();
            modelsc.Close();

            //  选中 DropDownList
            addbodstitle.SelectedValue = ((HiddenField)e.Row.FindControl("bodstitle")).Value;

        }

想得到gridview1中某一列的值做为条件来取得gridview2中dropdownlist值。
但每次运行至粗体代码时报错：用户代码未处理 System.NullReferenceException
  Message="未将对象引用设置到对象的实例。"

我该怎样做呢？请赐教！！