pfSense

https://github.com/pfsense/pfsense/blob/master/src/etc/inc/util.inc#L3322-L3333:

function array_remove_duplicate($array, $field) {
    foreach ($array as $sub) {
        if (isset($sub[$field])) {
            $cmp[] = $sub[$field];
        }
    }
    $unique = array_unique(array_reverse($cmp, true));
    foreach ($unique as $k => $rien) {
        $new[] = $array[$k];
    }
    return $new;
}

$cmp is not initialized with an empty array, and if all conditions are failed, $cmp == null

see https://forum.netgate.com/topic/169564/with-2-6rc-warning-array_reverse-expects-parameter-1-to-be-array-null-given