Trying to edit the sheet XWiki/XWikiUserPreferencesSheet from the User Profiles plugin resolves in an empty editor. The same behaviour can be observed for other sheets of the plugin.
I can confirm that this was working in version 11.
To reproduce open the (hidden) sheet.
Using andvanced edit select "Wiki".
Confirm the force edit dialog.
An empty editor is presented. The title is not filled in as well.