Hi,
I would have thought that checking if Session was null would avoid any problem like you have reported.
This was a recent change inspired by these links:
http://haacked.com/archive/2009/04/02/csrf-webforms.aspx
http://msdn.microsoft.com/en-us/library/ms972969.aspx
Can you post the actual stack trace so I can see what error needs to be caught there?
In the meantime I assume if you enable session state in IIS it will resolve this error.
Best,
Joe