TypeError: Error #1009: Cannot access a property or method of a null object reference. Why do I have

That's not a Flash issue... it is a designer error.

The 1009 error indicates that one of the objects being targeted by your code is out of scope.  This could mean that the object....

- is declared but not instantiated

- doesn't have an instance name (or the instance name is mispelled)

- does not exist in the frame where that code is trying to talk to it

- is animated into place but is not assigned instance names in every keyframe for it

- is one of two or more consecutive keyframes of the same objects with no name assigned in the preceding frame(s).

There could also be other less obvious reasons.

If you go into your Publish Settings Flash section and select the option to Permit debugging, your error message should have a line number following the frame number which will help you isolate which object is involved.