i change the price and ...

You can change the line:
if($this->config->get('thumbnail') && count($variant->images)>1){
to:
if($this->config->get('thumbnail') && count(@$variant->images)>1){
in that file to remove the notice.

Please note that you should not run your website with the display_error setting on as may reveal security information to end users, such as file paths on your Web server, your database schema or other information.

On our end, we'll change the code there to avoid that notice.

Could you check that you have the line:
if(!empty($new) || (empty($results)&&!empty($main_ids))){
On line 427 of the same file ?

Are you sure that you're text editor displays the lines correctly?
On line 436, you should have that code:
foreach($element->characteristics as $characteristic){
and few lines before, you should have either :
if(!empty($new) || (empty($results)&&!empty($main_ids))){
or:
if(!empty($new) || empty($results)){

If that's the second case, you should change it to the first case to solve the problem.

You indeed have the old code on the line 427. You could redownload hikashop and install it over your previous version. That should fix the problem.