Hi,
Thank you for your feedback.
This happens because of an improvement we made in the 4.3.0 so that if you activate the "required" setting of a custom address field, the system will check that the custom field is filled in when the address is selected as a billing or shipping address on the checkout. Before the system was allowing customers to checkout even without the field filled in as long as the address had been entered before the required option had been activated.
And we didn't think of the state custom field, which is by default required and there are by default some countries without states.
So I've added a patch so that if the country doesn't have any states, even if the "required" setting of the custom field is activated, it will still validate the address.
You can download the install package of HikaShop on our website and install it on yours to get the patch.
Note that you can also just turn off the "required" setting of the state field. Unless you have the "Add a 'Please select' value" setting activated in the field, having the required setting on or off is the same (besides that extra check that we need to do to skip the field when no states are in the country if the setting is turned on).