There is no problem importing demo data and later customizing it, even if the theme is not registered.
The error you are getting must be caused by something else. Please send us some valid wp credentials in a private reply. So we can give it a look.
Where exactly are you getting this? Can you send a screenshot? I checked it again and there is no error showing. I successfully removed the redundant items from the current menu, saved and no problems whatsoever arised. See screenshot:https://postimg.cc/image/v1boeqi1p/
You can change the menu to use as main menu in Appearance > Menus.
I understand. The issue is due to the fact that this demo does not have any menu itself. So the script can’t find it and thus generating the error.
We will make sure to fix this in future updates. Thank you for noticing it!
The demo “Scroll Story” has only one page. The home page. I already set it up for you. Please check the site and let me know.
