OK, Jason Dufair was able to help me figure out how to reproduce this. 
This can be reproduced on both FileList and FileList2.  You need to look
for the 'SqueakDotOrg' server entry in the list.

At the moment it looks like there is a problem with the object
associated with entry.  The current code appears to be expecting that
the altUrl instance variable will have an http URL in it and it doesn't
like the fact that it is instead nil.  I don't know enough about this
stuff to say what the real problem is.

Ken