MultiSelect with update on a temporary table

Hi,

I'm attempting a modification to the SalesCreateReleaseOrder form with the intention that a user can filter the order lines and then using a multiselect, ask AX to create a release order with the sales quantity automatically pulled from the corresponding sales line.

So far, I have added a new field to SalesCreateReleaseOrderLineTmp, the temporary table that the form uses as a datasource called SalesLineSalesQty, which carries the SalesQty from the corresponding SalesLine entry. I now want the user to select one or more records and then click a button. This would then copy the value from SalesCreateReleaseOrderLineTmp.SalesLineSalesQty into SalesCreateReleaseOrderLineTmp.SalesQty.

However, when attempting to do this, I get an error - 

Cannot edit a record in Order lines (SalesCreateReleaseOrderLineTmp).
The record has never been selected.

The code that I'm using (on the form button) is:

void clicked()
{
    MultiSelectionHelper            selectionHelper = MultiSelectionHelper::construct();
    SalesCreateReleaseOrderLineTmp  orderLineTmp;

    super();

    selectionHelper.parmDataSource(SalesCreateReleaseOrderLineTmp_ds);
    orderLineTmp  = selectionHelper.getFirst();

    while (orderLineTmp){
        ttsbegin;
        orderLineTmp.selectForUpdate(true);
        orderLineTmp.SalesQty = orderLineTmp.SalesLineSalesQty;
        orderLineTmp.update();
        ttscommit;
        orderLineTmp = selectionHelper.getNext();
    }

    SalesCreateReleaseOrderLineTmp_ds.reread();
    SalesCreateReleaseOrderLineTmp_ds.refresh();

}

Is anyone able to assist please?