This focus issue is a tough one

Gidday guys

This one has been bugging me all day.

If I select a row in my datagrid, then click out of my AIR desktop app to another program, desktop etc, then click back into the AIR app, I get the error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

    at fl.controls::TextInput/setFocus()

    at fl.managers::FocusManager/activateHandler()

I set up the following to deselect the row, but the error still occurs:

data_grid.addEventListener(Event.DEACTIVATE, deselectRow, false, 0, true);

private function deselectRow(event:Event):void

{           

      data_grid.selectedIndex = null;

}

Anyone had this happen before and know of a fix?

Cheers