Hi,
I'm not entirely sure.
The cause is rooted in the compatibility of a plugin in HikaShop with Joomla 5.
The problem is that this plugin works fine on Joomla 5 on all the other websites and I was not able to reproduce the error anywhere else and no one reported the issue in the past 2 years since Joomla 5 betas started rolling out and we added the compatibility to it in HikaShop.
So, that tells me there is something else on your website which messes with it, but I'm not sure what it could be.
I've modified a bit the plugin so that it makes sure HikaShop is loaded before it is executed. That way it ensures the compatibility layer of HikaShop does its job before the plugin and makes sure the error can't happen, no matter what.