I spent a couple hours diagnosing a 500 Null error. There was little information to go on and 500 Null errors are hard to figure out in general. Let's hope someone is helped by this.
The error resulted on a Linux Machine using CF7.02 with no updaters. The code in question was a set of nested custom tags all linked together using cfassociate. I dug through the code looking for any such potential null reference evaluations and chopped out large blocks of code at a time to try get the error to go away. If I took out enough CFML code, the error went away. It really didn't matter at all WHICH code I took out, just that I took out some mysterious amount.
The total lines of code in the custom tagset was less than 500 with no loops or anything that would jack up the lines of code when evaluated so this REALLY threw me for a loop.
Eventually, I updated to the latest updator for CF7 and the error went away. Strangely, there was no mention in the list of updater fixes, but trust me, the error went away. If you are having a similar problem, use ColdFusion MX 7.02 Cumulative Hot Fix 3 or later.
If you similar stuff going on in your applications, there is a good chance the ColdFusion MX 7.02 Cumulative Hot Fix 3 will sort it out for you.