Hi,
You have a system plugin, named "hikashopfield" which has not been developed by ourself and which override the HikaShop class field using the HikaShop class field from HikaShop 1.5.8
That's why HikaShop can't load his own class field, because the class is already define in your system plugin.
By deactivate this plugin, the javascript problem will be fixed but, I don't know the other impacts. If you have a custom plugin for overriding the custom fields, it should be for a reason.
With HikaShop 2.x, there is new ways for class overrides by extending them. So the extends is more compatible with updates.
Regards,