clone items object 

BUGFIX clone the $items DataList object before performing the populateSortValues method on it, as we don't want it manipulated for the operations to follow.... which was causing the following error for me

ERROR [Notice]: Undefined index: 2
IN POST /admin/pages/edit/EditForm/field/Advertisments/reorder
Line 234 in /Users/sheadawson/htdocs/aecl-ss3/gridfieldextensions/code/GridFieldOrderableRows.php