AS 3 Question

I have a MC that uses an external .as file and works fine when it is published to a .swf

I am now calling that above mentioned swf file to run in my main movie (it did before I added the external .as) but it will not.

The error message I get when publishing the Main Movie is:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Snowflake/SetInitialProperties()
at Intro_fla::MainTimeline/makeItSnow()
at Intro_fla::MainTimeline/frame1()