Ugh, more leftovers from the 1.3 code rewrite. Let me count the ways I hate Python and it's lack of proper compilation checking.
I'll push a new release in a couple of minutes. That error you were about to get displayed was that you were about to point to your own Calibre library. This is a BAD thing to be doing (in fact the plugin wouldn't let you). If I did let you, it would erase any empty book entries you had and hence affect your Calibre database integrity.