[nist sip] Server Authentication Problem

Hi guys,
i've alredy posted with this problem, and i'm trying to test the
authentication code in jain-sip-applet-phone, but i don't understand
why i have an exception when trying to send the authentication to the
server.
The program give me an NullPointerException when the program try to
get the request with this istruction:

Request request = registerStatus.getRegisterTransaction().getRequest();

Ok, it's not possible to get the request and the program going in
NullPointerException, but what are the steps for create a send an
authentication request to the sip Proxy?
i hope you understand my problem with my beatiful english! ;-)
thanks all
bye

-- 
Centorame Massimiliano
Undergraduate Student of Computer Science University - Bologna (Italy)