Firstly, the Session shouldn't be extended - it ties up server resources, and can slow things to a crawl. That's way some hosting services reduce it from the default 20 minutes, and ignore requests to extend it!
If you are getting an "object reference is required..." error for that line, then you need to look at the variable HttpSessionState - it's value is null, and I'm not at all sure where you got it from...
Normally, the session timeout would be set in the web.config file, rather than in code: HttpSessionState.Timeout Property (System.Web.SessionState)
] but seriously, if you need longer persistance, you should probably be lookign to not rely on teh session byut use a combination of DB and cookies for storage instead. Extending the session is not a good idea.