Loading SWFs into another SWF with a scrollbar class imported from a folder

Question

I'm building a small Flash Projector file. I'm loading external SWFs into a main SWF. The external SWFs will contain a scrollbar (sometimes 2) imported from a folder. The scrollbar works fine in the external SWF, but when I try to all the SWF to the main SWF, I get the following error. I'm still a novice to AS3.

[object maschera_8] TypeError: Error #1009: Cannot access a property or method of a null object reference. at Scrollbar/init() at loaded_fla::MainTimeline/scInit() at flash.display::DisplayObjectContainer/addChild() at loaded_fla::MainTimeline/frame1()

Here's the source code for the Main SWF, followed by the source code for the external SWF:

//clickListener for oldSection =================================================================

this.addEventListener(MouseEvent.CLICK, clickListener0);

var oldSection = null;

// clickListener for the Products buttons ======================================================

function clickListener0(evt:Event) {

    // test for nav links -----------------------------
    if (evt.target.name == "blue_btn") {
        loadSection("loaded.swf");
    } else if (evt.target.name == "red_btn") {
        loadSection("red.swf");
    }

    function loadSection(filePath:String) {
        var url:URLRequest = new URLRequest(filePath);

        var ldr:Loader = new Loader();
        ldr.contentLoaderInfo.addEventListener(Event.COMPLETE, sectionLoadedListener);
        ldr.load(url);
    }
}

// sectionLoadedListener =======================================================================

function sectionLoadedListener(evt:Event) {
    var section = evt.target.content;

    if (oldSection) {
        this.removeChild(oldSection);
    }
    oldSection = section;

    this.addChild(section);
    //section.alpha=0;
    section.x=0;
    section.y=0;
    //TweenLite.to(section,0.5,{x:-537.9, y:-189.5, alpha:1, ease:Quart.easeInOut});

}

External SWF source code:

import Scrollbar;

var sc:Scrollbar=new Scrollbar(scroll_mc.text,scroll_mc.maskmc,scroll_mc.scrollbar.ruler,scroll_mc.scrollbar.background,scroll_mc.area,true,6);
sc.addEventListener(Event.ADDED, scInit);
addChild(sc);

function scInit(e:Event):void {
    sc.init();
}