I have added a new Dexterity content type on Plone 5.0.6.
While creating this new type, I chose "no content type" under "containded items"
But now, this new type works like a folder. Of course nothing can be added to this folder. What is the problem?
When I clone the file type, this problem doesn't occur.
There is no error in this apart an usability problem: every content type created TTW is folderish (I'm not happy with this default too) but I think you can easily fix it manually.
go to ZMI
look for portal_types tool
select you content type
change "Content type class" from plone.dexterity.content.Container to plone.dexterity.content.Item
Thx @zopyx. I added my thoughts to that issue. I'd like to tell you that I could try to file a PR for it but I won't make silly promises. The logic David suggests seems a good solution, better than a checkbox.