Works by itself but wont when loaded

Question

ah, i cant figure this out
i have a slide to control the volume of a song it links to that autoplays when it starts
its a blank .fla with the class Volume and an as3 file called Volume with all the codes
by itself, it works perfect, but when i go to my index.fla where i try to use a UILoader to load the blank swf called volume
it says:
" TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at Volume() "
what do you think could be happening?

Answer 1

im not using code on the index.fla for it i put a UI Loader and put the folderadress in the source blank in its properties,
i mean, i do have code in the index just not having anything to do with the UILoader or its source. the UI loader is in frame 1, wich i named "musicframe"
the only other action i have in that frame is on a layer above and its only a stop().
the code for the as file class "Volume" is
package
    import flash.display.Sprite;
    import flash.display.Graphics;
    import flash.events.MouseEvent;
    import flash.events.Event;
    import flash.net.URLRequest;
    import flash.media.Sound;
    import flash.media.SoundChannel;
    import flash.media.SoundTransform;
    import flash.geom.Rectangle;
    public class Volume extends Sprite
        public var snd:Sound = new Sound();
        public var channel:SoundChannel = new SoundChannel();
        //Make sure you pass URLRequest an audio file on your computer.
        public var req:URLRequest = new URLRequest("underwater.mp3");
        public var boundary:Rectangle;
        public var sprite:Sprite;
        public var slider:Sprite;
        public var xPos:Number = stage.stageWidth / 2;
        public var yPos:Number = stage.stageHeight / 2;
        public var vol:Number;
                        Our request is loaded into the sound object and plays through
                        our channel. Volume is initially set at 50% and passed as a
                        transformation to our our channels soundTransform property
                        (a fancy way of saying volume). The init() function is called. 
        public function Volume()
            snd.load(req);
            channel = snd.play();
            vol = .5;
            channel.soundTransform = new SoundTransform(vol);
            init();
                        The init function creates and draws a rectangle and circle
                        to the stage and centers them based on the height and
                        width of the stage. In addition, a rectangle object is
                        created to 'contain' the sliding circle, like an imaginary box.
                        We pass -100 as the x value because it is added relative
                        to our sprite. If we set its x value at 0, or the sprites default x
                        value,the boundary would stop and start at the slider sprite. Change
                        -100 to 0 in the rectangle object to get a better idea of its use.
        public function init():void
            sprite = new Sprite();
            sprite.graphics.beginFill(0x666666);
            sprite.graphics.drawRect(xPos,yPos,200,2);
            sprite.graphics.endFill();
            addChild(sprite);
            sprite.x -=  sprite.width / 2;
            slider = new Sprite();
            slider.graphics.beginFill(0x999999);
            slider.graphics.drawCircle(xPos,yPos, 10);
            slider.graphics.endFill();
            addChild(slider);
            slider.addEventListener(MouseEvent.MOUSE_DOWN, dragSlider);
            stage.addEventListener(MouseEvent.MOUSE_UP, stopSlider);
            boundary = new Rectangle(-100,0,200,0);
                        dragSlider runs when the use holds the mouse button down. A
                        startDrag method is used on our sprite where we specify boundary
                        as our dragging limits. A new event handler designed
                        to change the mouse volume is subsequenlty called per frame, where
                        the slider.x property determines volume.
        public function dragSlider(event:MouseEvent):void
            slider.startDrag(false,boundary);
            slider.removeEventListener(MouseEvent.CLICK, dragSlider);
            slider.addEventListener(Event.ENTER_FRAME, changeVolume);
                        Stops dragging and removes the event listener to save on space. Again,
                        volume will be based on the sliders current x position, which is
                        constantly being recalculated per frame because we used an
                        ENTER_FRAME event.
        public function stopSlider(event:MouseEvent):void
            slider.stopDrag();
            slider.removeEventListener(MouseEvent.MOUSE_UP, stopSlider);
                        This function is constantly recalculating the vol variable
                        based on the sliders x position, relative to the length of
                        our rectangle. Creates a decimal range from 0 to 1, where 1
                        represents 100% volume and 0 represents mute. Anything exceeding
                        100% causes distortion.
        public function changeVolume(event:Event):void
            vol = .5 + Math.round(slider.x) / 200;
            channel.soundTransform = new SoundTransform(vol);
this as file i use as "volumecontrol" 's class, wich is a blank fla. with only one blank frame.  by itself it works perfect :S
and this is everything that shows on the output panel, after debugin permited is checked:
" TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at Volume() "
please, enlighten me x_x