I've found two small problems in the XIFF library.
The first one is in the XMPPConnection.getRegistrationFields_result, it looks like this method does not dispatch an event while it should. I think I fixed it. However, I'm not sure about this one.
The second one is in RegisterExtension.key. This returns a property of the myKeyNode propery, even though this could be null. If it is null this throws an error, I fixed this too.
The patch file is attached.