Can you post a log of the outgoing SOAP request you're making (with
your confidential token and password information X-ed out)? It looks
like you're making some sort of call that requires an AdGroup object,
and you're setting the id of that object to null (rather than leaving
out the id field, or setting it to a long) and that's causing the
exception. Looking at the SOAP request would confirm this.
Cheers,
-Jeff Posnick, AdWords API Team