[Containment Appearance] Make sure to always load a config view
If StackView is empty, currentItem will be null and we would break all over the place.
This can happen when you have a wallpaper without config, so "sourceFile" would stay empty
and thus onSourceFileChanged wouldn't be triggered and we wouldn't load anything into our StackView.
Differential Revision: https://phabricator.kde.org/D3911