As explained in the other thread, this happens because there is a problem with the root category of your website. This category should never be changed (and actually cannot be changed via the interface of HikaShop).
The fix Jerome provided only avoid the query to crash, but that query should never crash since the system should always find the root category in the hikashop_category table. A proper fix in your case is to restore the root category to how it was before, after the installation of HikaShop, so taht all the bits which use it in HikaShop can use it and function properly.