You need to declare "$getPages" outside the IF statement on line 25 of next_page_not_next_post.php. i.e. insert the following line before the IF statement:
$getPages = null;
Otherwise when you have PHP set to report errors (like when WP is in debug mode), then it'll issue a warning when $getPages isn't yet declared on line 40.
I've added the fix to my copy and it works fine under WP 3.4.1.