You're the second person to report an error occurring on that line.
The only way I know to get that error is to click the plug in with no books selected (or choose 'Add New from URL(s)'), then *not* enter any URLs and click OK.
(Which is a case I should start checking for, but haven't yet.)
If you could start calibre in debug mode (calibre-debug -g) and post the debug output, that would be helpful.
Or if it happens consistently on the same stories, knowing the URLs for those stories would help me a lot.