Add-on failed with exception; Event Type 16

Hi,

I am trying to add a new column in the Matrix which is displayed in the Sales Order form. When I run this addon, I get the error Add-on 9000016 failed with exception; Event Type 16.

My code is:

private void SBO_Application_ItemEvent( string FormUID, ref SAPbouiCOM.ItemEvent pVal, out bool BubbleEvent ) {

            BubbleEvent = true;

            if ( ( ( pVal.FormType == 139 & pVal.EventType != SAPbouiCOM.BoEventTypes.et_FORM_UNLOAD ) & ( pVal.Before_Action == true ) ) ) {

                // get the event sending form

                oOrderForm = SBO_Application.Forms.GetFormByTypeAndCount( pVal.FormType, pVal.FormTypeCount );

                if ( ( ( pVal.EventType == SAPbouiCOM.BoEventTypes.et_FORM_LOAD ) & ( pVal.Before_Action == true ) ) ) {

                    SAPbouiCOM.Item formItem = null;

                    //Get the Matrix

                    formItem = oOrderForm.Items.Item("38");

                    SAPbouiCOM.IMatrix oMatrix = (SAPbouiCOM.IMatrix)formItem.Specific;

                    SAPbouiCOM.IColumn col1 = null;

                    SAPbouiCOM.IColumn colItem = null;

                    col1 = oMatrix.Columns.Add("RKMatrix001", SAPbouiCOM.BoFormItemTypes.it_EDIT);

                    col1.TitleObject.Caption = "RKColCaption";

                    col1.Description = "RKColDesc";

                    col1.Width = 30;

                    col1.DisplayDesc = true;

                    //"5" is for Mfr Serial No.

                    //col1.DataBind.SetBound(true, oMatrix.Columns.Item("5").DataBind.TableName, oMatrix.Columns.Item("5").DataBind.Alias);

                    SAPbouiCOM.UserDataSource oUserDataSource = oOrderForm.DataSources.UserDataSources.Add("RK_DS01", SAPbouiCOM.BoDataType.dt_SHORT_TEXT, 20);

                    col1.DataBind.SetBound(true, "", "RK_DS01");

                    oUserDataSource.Value = "TEST VALUE";

                    col1.Editable = true;

                }

            }

        }

What am I doing wrong?

Thanks.