As several users found out, we had some recent trouble with exceptions being thrown during downloads. A combination of insufficient error handling in the application and some changes in the data we were processing caused the error. The 1.0.1 release should fix the exceptions by preventing the UI from trying to update the progress bar if we don't have download progress to report.
In the meantime it looks like 1.0 may be functioning alright at least part of the time. I'm not sure what's happened to cause this issue to appear, but we'll make sure if anything else weird happens that we stay on top of it. Thanks to those users who reported the problems!