Hi,
Please try to add the code:
else
$payment->taxes[$tax->tax_namekey] = new stdClass();
after the code:
if(is_object($tax))
$payment->taxes[$tax->tax_namekey] = clone($tax);
in the file administrator/components/com_hikashop/classes/currency.php and it should hopefully fix the warning message.
Please report the result so that we can add the modification for the next version of HikaShop.
That way you won't have to redo the modification after the next update and others won't get the warning in the future.