Had the same issue. if you go to /wp-content/plugins/advanced-custom-fields/core/api.php, and find line 292…
REPLACE:
array_multisort($menu_order, SORT_ASC, $array);
WITH THIS:
if (isset($menu_order)):
array_multisort($menu_order, SORT_ASC, $array);
endif;
That should stop the error.
