Hide the ShadowRectangle for non-loaded previews
This ommission is exposed when these views are used with a dark colour scheme, as shown in the bug report https://bugs.kde.org/show_bug.cgi?id=422048
This ommission is exposed when these views are used with a dark colour scheme, as shown in the bug report https://bugs.kde.org/show_bug.cgi?id=422048