Hi,
Change the line:
$query = 'SELECT category_id FROM '.hikashop_table('category').' WHERE category_type=\'product\' AND category_parent_id='.$root.' LIMIT 1';
to:
$query = 'SELECT category_id FROM '.hikashop_table('category').' WHERE category_type=\'product\' AND category_parent_id='.(int)$root.' LIMIT 1';
in the file administrator/components/com_hikashop/views/config/view.html.php and that will remove the error.
This error indicates that the root category has been deleted, and that shouldn't happen normally. You should click on the "check database" button in order to fix that.