How to load a movieclip attached to a class from the Library?

programming flash
1

I’m at a dead end but I feel like the answer is right in front of me… I’m hoping someone can shed some light on this.

I’ve created a simple class that defines a scrollbar to scroll content. I’ve applied this class (ScrollBox.as) to a movieclip in the library as the Base Class.

Now, I’d like to add this movieclip to the stage with AS3. So for the Linkage, I’ve given the movieclip a class name of ‘AboutBox’. This is the code I’ve written to add it to the display list:

var aboutBox:AboutBox = new AboutBox();
aboutBox.x = 65
aboutBox.y = 179
addChild(aboutBox);

When I test the swf, I get this error:

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

What am I missing here?

thanks