If the error you're seeing has the string “isMediaURL” somewhere in the middle of it, that means you probably have a Core Wrapper Failure. If you got here from a link in the application, you definitely had a Core Wrapper Failure.
A Core Wrapper Failure happens when the internal playback core doesn't initialize properly or decides it doesn't want to play nice anymore and storms away mad.
On Linux, this means your gstreamer is likely not installed properly. Please see Setting Up GStreamer for troubleshooting your GStreamer set up.
We're not sure why a few people are seeing this on Windows, yet, but it undoubtedly has something to do with VLC. Maybe something to do with having the VLC player also installed on your machine.
We haven't yet seen this bug on the Mac. That's good. If you get here because the program barfed, that's bad. Please file a new bug.
If you didn't have a Core Wrapper Failure, you should just go to our issue tracker and see if your bug is already posted. If not, please do make a new one.
Good luck, soldiers!