Steps to reproduce:

1. Access PlatformUI;

2. Go to "Admin Panel -> Content types";

3. Click on content type group "Content";

4. At the bottom of the content types list, click on "Create a content type";

5. Give the proper specs to the content type that you need;

6. Click "OK" at the bottom;

7. Go to "Content -> Content structure -> Content Tree";

8. Click on "Create" on the rightside menu;

9. Choose the content type you just created;

10. Name it and publish it;

11. Delete the content you just created;

12. Go back to "Admin Panel -> Content types" and into content type group "Content";

13. Verify that the "Delete" button affected to the content type you just created is NOT greyed out and IS clickable;
open firebug or similar tool and go to Console;

14. Click on "Delete" to remove the content type.
You'll see the error notification:

Argument '$contentTypeId' has a bad state: ContentType with given id still has content instances and therefore can't be deleted 

and on the Console:

POST http://ezp5.rui/pjax/contenttype/delete/43           403 Forbidden  		246ms                 io-base-min.js (line 8)
"NetworkError: 403 Forbidden - http://ezp5.rui/pjax/contenttype/delete/43"                                                      43

This happens because the removal of the Content Type doesn't take into account that the Content you deleted is in trash state, and so far, in PlatformUI, removed items sent into Trash cannot be expunged from the trash, and they are so far, by default, always sent to Trash when removed from PlatformUI.