A quick search brought up that 'NullTextureSet' is, well, a texture set. I think it's unrelated.
Also, you may have an outdated GECK version. I downloaded 1.1 and it wouldn't load any files; I swear I went to the
exact same download link, and it downloaded 1.3 instead. It's working now for me.
Try to replace your current GECK with 1.3 if you suspect that that is the problem.
http://fallout.gamesas.com/eng/downloads/updates-GECK-fnv-eula.php