@Override
    public Point getToolTipLocation(MouseEvent event){

        int col = columnAtPoint( event.getPoint() );
        int row = rowAtPoint( event.getPoint() );
        if (row >= 0 && col >= 0 && getCellToolTip(event) != null) {
            return getCellRect(row, col, false).getLocation();
        }
        return super.getToolTipLocation(event);
    }