Hi,
This happens for several reasons:
- you've apparently set the "Clean cart when order is" setting to "created" in the HikaShop configuration. Set it to "confirmed" and you should be redirected back to the checkout.
- Since the cart is cleared, you're redirected to the "URL where to redirect when cart is empty" from your HikaShop configuration. But since that setting is empty, HikaShop tried to generate the URL itself. I would recommend configuring the main URL of your shop in that setting.
- That "Call to a member function get() on null" error is a generic message. Please activate the "error reporting" and "debug mode" setting of your Joomla configuration and that will display more information with that message, which will allow you/us to know where that error is coming from and why it doesn't work.