Loading .swfs into a movieclip, same level?

Hi,

I am a beginner in flash and i jumped straight into learning as3, therefore would appreciate any help with my problems. Thanks!

My problem is on my stage i have a loader_mc invisible movieclip that i want to load .swfs into using addchild

Problem is, everytime i click on a button that addchild the .swf keeps adding ontop like layers, therefore in my function button i tried to removechild before addchild but i will keep getting error #1009

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

I was wondering if anyone can teach me any other better ways how to load movies like in as2, on the same level when the button is clicked, the previous .swf is kicked aside and the new one will load.

Thanks alot!!