Dynamically create ComboBox in ASP.Net

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ASP.NET

Dear All,
I am using below code to create ComboBox dynamically. But in page load i am getting an error "Sys.ArgumentException: Value must not be null for Controls and Behaviors. Parameter name: element..."
I am using VS 2010 and AJAX 4.0 toolkit.

foreach (DataRow dR in dtCommitmentRowSource.Rows)
        {
            row = new TableRow();
            ddl = new AjaxControlToolkit.ComboBox();
            ddl.Visible = true;
            ddl.RenderMode = AjaxControlToolkit.ComboBoxRenderMode.Block;
            //1st Column
            cell = new TableCell();
            cell.CssClass = "tdVerticalHeader";
            cell.Text = string.Format("{0} Code", dR["LevelName"]);
            cell.HorizontalAlign = HorizontalAlign.Left;
            row.Cells.Add(cell);
            //2nd Column
            ddl.CssClass = "WindowStyle";
            ddl.ID = dR["LevelName"].ToString();
            ddl.DataSource = c.getCOACodesByCOAStructure(Convert.ToInt32(dR["RecordID"]));
            ddl.DataTextField = "Code";
            ddl.DataValueField = "RecordID";
            ddl.AutoCompleteMode = AjaxControlToolkit.ComboBoxAutoCompleteMode.SuggestAppend;
            ddl.AutoPostBack = true;
            ddl.DataBind();
            cell = new TableCell();
            cell.Width = new Unit(100, UnitType.Pixel);
            cell.CssClass = "tdsLevelTxs";
            cell.Controls.Add(ddl);
            row.Cells.Add(cell);
            //3rd Column
            cell = new TableCell();
            cell.CssClass = "tdsLevelTxs";
            cell.Text = "Code Desc Here";
            row.Cells.Add(cell);
            tbSubCommitment.Rows.Add(row);
            row = new TableRow();
            cell = new TableCell();
            cell.CssClass = "tdHorizentalEmpty";
            row.Cells.Add(cell);
            tbSubCommitment.Rows.Add(row);
        }

I found that the ComboBox does not get any datasource.

I dont know where the error "Sys.ArgumentException: Value must not be null for Controls and Behaviors. Parameter name: element..." comes from.