After configuration of a new pagetype and adding a page the 'composer' view is empty.

Console shows error 500 / response:

{"error":{"message":"Argument 1 passed to Concrete\\Core\\Application\\Service\\Composer::displayButtons() must be an instance of PageType, instance of Concrete\\Core\\Page\\Type\\Type given, called in \/Users\/user1\/Documents\/WebPHP7\/project\/concrete\/views\/panels\/details\/page\/composer.php on line 29"},"errors":["Argument 1 passed to Concrete\\Core\\Application\\Service\\Composer::displayButtons() must be an instance of PageType, instance of Concrete\\Core\\Page\\Type\\Type given, called in \/Users\/user1\/Documents\/WebPHP7\/project\/concrete\/views\/panels\/details\/page\/composer.php on line 29"]}

I tested this on a fresh installation on php7.1.8 with 8.4.0RC4
Tested this on several environments. This only happens to new configured pagetypes. Existing pagetypes are working as expected.

Steps to reproduce on a fresh installation:

Visit: /dashboard/pages/types and add a new pagetype

Settings I made:
Page Type Name: example
Page Type Handle: example
Default Page Template: Right Sidebar
Launch in Composer: Yes
Is this page type frequently added: Yes
Allowed page templates: Selected Page Templates -> Right Sidebar
Publish Method: Choose from all pages when publishing
Selector Form Factor: Sitemap Pop-Up
[ADD]

After that I hit the [edit form] button for this page type.
The I hit [Add set] and give the name 'settings'
For this set I add a built in property 'page name'
That's enough for reproducing the issue

Now I add this new page. And the page opens with a blank composer view. Instead of showing me the page-name in the set 'settings'.