Using Forge 1.6.4-Forge9.11.0.883, Minecraft 1.6.4, and InventoryTweaks-MC1.6.2-1.56-b77, I get an NPE on world load.
Pastebin here: http://pastebin.com/HyefCMvs.

Also got the NPE using forge 1.6.4-forge.9.11.0.965.

Either your configuration files are messed up and failed to load, or see #153

1.6.4 versions don't handle this situation very well