Fixes "Notice: Undefined variable: grid in app/code/community/BL/CustomGrid/Model/Grid/Type/Tax/Class.php on line 38"

see https://github.com/ho-nl/mage-enhanced-admin-grids/commit/9f83a09ba49cdaba5de302f05b8986223d080e47
Fix strict notice caused by passing the return value of a function into array_pop which expects an array reference.