It looks like your old db may be a little out of whack and there is a permission entry with no user. I'll add a fix to check for this. Line 1180 of threads_import.php looks like this:

Code
if (!isset($user_array[$perm['P_Uid']])) {
	$new_uid = get_new_uid($perm['P_Uid']);
} else {
	$new_uid = $user_array[$perm['P_Uid']];
}

Right after that, add this:

if (!$new_uid) continuel