AS3: AddChild issue -- "TypeError: Error #2007: Parameter child must be non-null."

My following code gave me TypeError: Error #2007: Parameter child must be non-null runtime error. not sure why...I would appreciate any help...

        mySb = new ScrollBar(); 
        mySb.x = cont.x; //+ cont.width;
        mySb.y = cont.y;
        mySb.height = contMask.height;
        mySb.enabled = true;
        addChild(mySb); 

Updated

package com.search.view

{

import com.search.events.YouTubeSearchEvent;

import fl.controls.ScrollBar;
import fl.controls.Slider;
import fl.controls.UIScrollBar;
import fl.events.ScrollEvent;
import fl.events.SliderEvent;

import flash.display.Shape;
import flash.display.Sprite;
import flash.events.MouseEvent;
import flash.geom.Rectangle;
import flash.net.URLLoader;

public class SearchResultContainer extends Sprite
{
    private var cont:videoCont;
    private var contMask:Sprite;
    private var mySb:ScrollBar;


    public function SearchResultContainer()
    {
        super();

        }
    public function get selectedVideoID():String{
        return newVideoID;
    } 

    public function createContainer(_x:Number,_y:Number, videoResult:Array):void{



    cont=new videoCont();
    cont.x=_x;
    cont.y=_y;
    addChild(cont);

    contMask = new Sprite();
    contMask.x = cont.x;
    contMask.y = cont.y;
    createMask(contMask,0x000000,452,88);

    addChild(contMask);     
    cont.mask = contMask;


    mySb = new ScrollBar(); 
            mySb.x = cont.x; //+ cont.width;
            mySb.y = cont.y;
            mySb.height = contMask.height;
            mySb.enabled = true;
            addChild(mySb); //problem code here...




    }


    private function createMask(inSrc:*,inColor:Number=0x999999,inW:Number=80,inH:Number=50):void{
        var rect:Shape=new Shape();
        rect.graphics.clear();
        rect.graphics.beginFill(inColor);
        rect.graphics.drawRe开发者_开发问答ct(0,0,inW,inH);
        rect.graphics.endFill();
        inSrc.addChild(rect);
    }



}

}

I am in Flex environment....

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Try to add a breakpoint before the problem occurs and check the mySb value , it looks like it's probably null , if it's not you'll have to look for null values either in the DisplayObjects you're using or the properties you're assigning them... if it is null , maybe you need to set more properties to your ScrollBar instance before adding it to the display list...

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In my case I solved it adding the component to the movie library, but working in Flash CS5.5 environment.