Hello,
The error is not related to HikaShop but it's related to HikaSerial 2.0.0 and the feature which allow to download the serials directly from the order listing.
I was able to reproduce it by perform a search which gave me an empty listing.
So you need to edit the file "administrator/components/com_hikaserial/classes/order.php" and replace the line
$order_ids = array_keys($view->rows);
By
$order_ids = array_keys($view->rows);
if(empty($order_ids))
return;
We will update the HikaSerial package with that patch as soon as possible.
Regards,