Cannot reproduce the issue here on linux, but have the feeling windows builds are much
more easily broken with referencing bad data from bpy. So here guessing direct loop
over bpy.data.xxx when you do add and remove stuff from said xxx inside the loop
is not a good idea - which seems logical in the end. :P
Not much to add, you can now clear previews from current .blend file, or a set of non-opened files.
Likewise, you can generate previews (for mat/tex, objects, groups, scenes, ...).
