After troubleshoot long hour ago, it seems like that is related with your current template issue based on what i observation.

One thing is when I switch to use Protostar template, it work perfectly fine.

So I try to create a menu item for Joomla single category menu, you can access this page (https://replaceyoursite.com/testmenu) but you have to login as superadmin user.

After that, when you access this page and click on the "end" button which is url contain ?start=60 , whenever the pagination contain ?start=60, it will always hit this error even Joomla page also like this.

Error Cannot access empty property, Cannot access empty property

It would be best if you can consult with your current template provider regarding this.

Thank you for looking into it Arlex,

Seems strange, as you say anything ending ?start=60 or ?start=61 gives that error but from ?start=62 onwards all is fine

You're welcome.

Yes that was strange, but i didn't test on end of ?start=62 and ?start=61, perhaps you can check with your template provider and see how it goes.

Because Joomla single category page also hitting the same issue (https://replaceyoursite.com/testmenu)