Linking class files

Hi, i’m pretty new AS3, and i’m having problems linking a class file to my main document class. The class I want to link is a sound based class, and has play,pause buttons aswell as a volume slider, it is also linked to a mp3 file using URLRequest. When linking the sound class file(SoundSlider.as) to my main class document(Portfolio.as) I get the following error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at SoundSlider$iinit()
at Portfolio$iinit()

When attempting to link the two classes, I first added the import SoundSlider; statement to my main class, I also declared and made an instance of it,

var _soundslider:SoundSlider = new SoundSlider;

I then added used addChild(_soundslider) to add it onto the stage. All the files are in the same folder. Any help would be greatly appreciated.

Many Thanks
benny