TypeError: Error #2007

Here's my error

TypeError: Error #2007: Parameter child must be non-null.

    at flash.display::DisplayObjectContainer/addChildAt()

    at fl.controls::BaseButton/drawBackground()

    at fl.controls::BaseButton/draw()

    at fl.core::UIComponent/callLaterDispatcher()

here's my AS

import fl.events.SliderEvent;

import flash.geom.ColorTransform;

import flash.events.Event;

s_red.addEventListener(SliderEvent.THUMB_DRAG,changeColor)

s_green.addEventListener(SliderEvent.THUMB_DRAG,changeColor)

s_blue.addEventListener(SliderEvent.THUMB_DRAG,changeColor)

function changeColor(Event:SliderEvent):void{

    var r= s_red.value;//0- 255

    var red = r.toString(16);//number to hexcode conversion with radix set to 16

    if(red.length<2){red ="o"+red}//if its a single digit number make it double by prefixing ''o''

    var g= s_green.value;//0- 255

    var green = g.toString(16);

    if(green.length<2){green ="o"+green}//if its a single digit number make it double by prefixing ''o''

    var b= s_blue.value;//0- 255

    var blue = b.toString(16);

    if(blue.length<2){blue ="o"+blue}//if its a single digit number make it double by prefixing ''o''

    var myColor:ColorTransform =new ColorTransform();

    myColor.color = uint("ox"+red+green+blue);

    mc.transform.colorTransform = myColor;

    rgb.text = red+green+blue;

}

    myColorfield.addEventListener(Event.CHANGE,changeColor1)

function changeColor1(event:Event):void{

    var myColor1:ColorTransform = new ColorTransform();

    myColor1.color = uint("ox"+myColorfield.text);

    if(myColorfield.length==6){mc.transform.colorTransform = myColor1}

}

Help please