Hi,
I'm not able to reproduce the issue with the same versions of HikaShop and Joomla on my end.
Your stack trace seems to indicate that the error comes from the line:
include $this->_template;
in the file /libraries/src/MVC/View/HtmlView.php of Joomla because $this->_template is null.
However, reading the code there, $this->_template is set by the line:
$this->_template = Path::find($this->_path['template'], $filetofind);
a few lines before at the same place.
And reading the code of the Path::find function ( in /libraries/src/Filesystem/Path.php ) it can only return false or a string. It can't return null like the error message says.
So even without going deeper into it, I don't see how that's possible.
It would seem to indicate that it can't find the cart / form.php view file (used when creating a new cart). But I don't see why.
We would need to do some debugging directly there to be able to understand what's going on.
Could you provide a backend and FTP access in order to look into it ?
If so, please use our contact form to provide them along with a link to this thread for reference:
www.hikashop.com/support/contact-us.html