Need help with GridExtensionPack

I oppologize in advance for all the posts I have had recently, but these client side extensions are killing me.

Currently, my task is simple. When somone has a row selected in the grid and they press UP or DOWN, it needs to go up a row or down a row and SELECT the row. If the editor is opened, it needs to save the data, close the editor, then move up/down respectively. This seems reasonably simple, and I thought I understood what I need to do.

I downloaded the sample project from here, and began changing things around a bit. Everything has been working well, until I added the following code to the extend() function in the EditorHandlingOverrideExtensionConnector class:

grid.addDomHandler(new KeyDownHandler() {
           @Override
           public void onKeyDown(KeyDownEvent event) {
               JsonObject selectedRow = grid.getSelectedRow();
               if(event.getNativeKeyCode()== KeyCodes.KEY_DOWN ){
                   for (int i = 0; i < grid.getDataSource().size(); i++) {
                       JsonObject tRow = grid.getDataSource().getRow(i);
                       if (selectedRow.equals(tRow)) {
                           grid.select(grid.getDataSource().getRow(row + 1));
                           grid.select(grid.getSelectedRow());
                           break;
                       }
                   }
               }else if(event.getNativeKeyCode()== KeyCodes.KEY_UP ){
                   for (int i = 0; i < grid.getDataSource().size(); i++) {
                       JsonObject tRow = grid.getDataSource().getRow(i);
                       if (selectedRow.equals(tRow)) {
                           grid.select(grid.getDataSource().getRow(row - 1));
                           grid.select(grid.getSelectedRow());
                           break;
                       }
                   }
               }
           }
       }
    , KeyDownEvent.getType());

I understand that this code isn't bulletproof, it is simply supposed to work when the editor is closed and they press up/down. However, once I press either the up/down arrow, I get the following exception:

This client-side selection model SingleSelectionModel does not know selected row.java.lang.UnsupportedOperationException: This client-side selection model SingleSelectionModel does not know selected row.
   at Unknown.$fillInStackTrace(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.Exception(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.RuntimeException_0(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.UnsupportedOperationException_0(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.$getSelectedRow(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.getSelectedRow(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.$getSelectedRow_0(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.handleMoveEvent_0(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.$handleEvent_0(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.onBrowserEvent_49(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.dispatchEvent_4(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.dispatchEvent_6(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.apply_0(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.entry0(org.vaadin.customgrid.CustomGridWidgetset-0.js)
   at Unknown.<anonymous>(org.vaadin.customgrid.CustomGridWidgetset-0.js)

I don't understand why. The grid.select function is copied from the example. Is this just not possible in the DomHandler? If so, what do I do here? 

Please help ...