I'm having the exact same problem as the OP in this thread: http://www.sitefinity.com/devnet/forums/sitefinity-3-x/set-up-installation/null-reference-with-active-directory-provider.aspx#1205193
I've added the ADRoleProviderTest code to my App_Code folder, but it doesn't change the error at all, nor does it log an error in the event viewer. I've probably done something wrong, but the instructions are too vague for this newbie to follow along.
I've been implementing Gabe Summer's code as I followed along in his webcast to the place where he changed the Role and Memebership providers to use AD, and that's when I got the error. The only difference is I've been using our actual corporate AD, not the one in the VM.
So how do I troubleshoot this thing? I wish there was local support for Sitefinity; I'd pay to have someone come out and set this up the way we need it.