Problem of addFacet/setFixedFacetValue in CubeGrid

[addy_hu]

I use the sample of "basic CubeGrid" by adding three listGrid: origin list, row list and column list. The user can hide the facet that he doesn't want to see by dragging it to the origin list; and can review the facet by dragging it to the row or column list. The code is:

public class BasicCubeGrid implements EntryPoint {

    @Override
    public void onModuleLoad() {
        ListGrid originList = createDragListGrid();
        originList.setFields(new ListGridField("title", "Origin"));

        ListGridRecord region = new ListGridRecord();
        region.setAttribute("id", "region");
        region.setAttribute("title", "Region");
        
        final ListGrid rowList = createDragListGrid();
        rowList.setFields(new ListGridField("title", "Row"));
        rowList.setData(new Record[] { region });

        final ListGrid colList = createDragListGrid();
        colList.setFields(new ListGridField("title", "Column"));

        final CubeGrid cubeGrid = new CubeGrid();
        cubeGrid.setData(ProductRevenueData.getData());

        cubeGrid.setWidth100();
        cubeGrid.setHeight100();
        cubeGrid.setHideEmptyFacetValues(true);
        cubeGrid.setShowCellContextMenus(true);

        final NumberFormat numberFormat = NumberFormat.getFormat("0,000");

        cubeGrid.setCellFormatter(new CellFormatter() {
            @Override
            public String format(Object value, ListGridRecord record, int rowNum, int colNum) {
                if (value == null)
                    return null;
                try {
                    return numberFormat.format(((Number) value).longValue());
                } catch (Exception e) {
                    return value.toString();
                }
            }
        });

        cubeGrid.setColumnFacets("quarter", "month", "metric");
        cubeGrid.setRowFacets("region", "product");

        originList.addRecordDropHandler(new RecordDropHandler() {

            @Override
            public void onRecordDrop(RecordDropEvent event) {
                String facetId = event.getDropRecords()[0].getAttribute("id");

                cubeGrid.setFixedFacetValue(facetId, "Eastern U.S.");

            }
        });

        rowList.addRecordDropHandler(new RecordDropHandler() {

            @Override
            public void onRecordDrop(RecordDropEvent event) {

                String facetId = event.getDropRecords()[0].getAttribute("id");

                if (cubeGrid.getFacet(facetId) != null) {
                    cubeGrid.addRowFacet(facetId);
                }
            }

        });

        colList.addRecordDropHandler(new RecordDropHandler() {

            @Override
            public void onRecordDrop(RecordDropEvent event) {

                String facetId = event.getDropRecords()[0].getAttribute("id");

                if (cubeGrid.getFacet(facetId) != null) {
                    cubeGrid.addColumnFacet(facetId);

                }
            }

        });

        HStack stack = new HStack();
        stack.setWidth100();
        stack.setHeight100();
        stack.setMembersMargin(20);
        stack.addMember(originList);
        stack.addMember(rowList);
        stack.addMember(colList);
        stack.addMember(cubeGrid);
        stack.draw();

    }

    private ListGrid createDragListGrid() {

        ListGrid listGrid = new ListGrid();
        listGrid.setCanSort(false);
        listGrid.setShowSortArrow(SortArrow.NONE);

        listGrid.setSelectionType(SelectionStyle.SINGLE);

        listGrid.setCanDragRecordsOut(true);
        listGrid.setCanAcceptDroppedRecords(true);
        listGrid.setCanReorderRecords(true);

        listGrid.setDragDataAction(DragDataAction.MOVE);
        listGrid.setHeight(100);
        return listGrid;
    }
  
}

First, the facet "Region" appears in the row list. I can drag it to the column list to show it as a column, then drag it to the origin list to hide it. Until now, it works fine; but then when I try to drag it to the row facet, a JS exception is thrown:
Caused by: com.google.gwt.core.client.JavaScriptException: (TypeError): this.rowFields[i][0] is undefined
fileName: http://127.0.0.1:8888/basicCubeGrid/sc/modules/ISC_Analytics.js
lineNumber: 445
I have checked that there is a fixedFacetValue on region when the addRowFacet() is called. But why it can not get the rowFields on region?
Thank you for any help.