improved fix for crashes due to calling getActiveCellPosition() (and other APIs which invoke absBox() under the hood) when either no activeCell has been set right now (activeCell == null) or when the activeCell has been detached from the DOM (which happens when you select nodes and scroll quickly in a large grid).

The current fix ALWAYS produces a BOX object for grid.getActiveCellPosition(): just like slickgrid does internally, you, the user, must check the .visible boolean to see if the top/left/width/height/etc. answer is actually useful.