Hello,
We are sorry for the disruption. We found the cause and we have a fix.
The error comes from the old Joomla class names (like JLanguage, JObject, JPlugin) that several HikaShop add-ons still use. On Joomla 6, when Joomla's own Backward Compatibility plugin is enabled, HikaShop was leaving it to that plugin to provide those names. But that plugin only sets them up later during the page load, while some elements can need them earlier in some cases. So the name was not defined yet at that moment, which is the "Value of type null is not callable" error you saw in helper.php, and the "class not found" errors related to HikaSerial.
HikaShop now sets these names up itself, early and in every case, so the update completes and the shop and the add-ons load normally. HikaSerial does not need any change.
Download the new install package of HikaShop on our website and install it on yours and it will add the patch and it should all work fine again.
We reproduced your exact configuration (Joomla 6, HikaShop 6.5.2, HikaSerial, with the compatibility plugin enabled), confirmed the crash, and confirmed that this fix removes it. It will also be included in the next HikaShop release.