Steps to reproduce the issue

Use a system-pagebreak in an article in J4 beta 1.

Expected result

As usual. Worked in J4 alpha 12.

Actual result

No article is displayed with the following message:
The requested page can't be found.

An error has occurred while processing your request.

You may not be able to visit this page because of:

an out-of-date bookmark/favourite
a mistyped address
a search engine that has an out-of-date listing for this site
you have no access to this page

Go to the Home Page

If difficulties persist, please contact the website administrator and report the error below.

0 Call to a member function isRtl() on null 

System information (as much as possible)

Webhosting with PHP 7.3. Just upgraded from alpha 12 to beta 1 and upgraded database tables using Joomla function.

Additional comments

The only way to avoid this is to disable the corresponding plugin (Content - Page Break). But the system-pagebreak does not work anymore.