Solving Error #2007: Parameter blendMode must be non-null.

Different versions of Flex's v3 SDK are not compatible with each other. You cannot always use a swc compiled against 3.3 in your 3.2 project. This is a useful warning if you're using multiple swc's from other projects.

The unobvious Flex error you get when mixing SDK versions is:

TypeError: Error #2007: Parameter blendMode must be non-null.

I didn't see this problem until my component tried creating a drop-down list, at which point it tried validating the list and spat this out.

I can only find this one reference to this problem, hence this post seems like it might help somebody:

Agile UI: I hate error messages like this