ActionScript 3.0 and Classes Help

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Flash

Hi, I am relatively new to object oriented programming in ActionScript 3.0..I designed some cloud and I am trying to control the movement of the cloud via a class. The code I have used is from a tutorial I followed online. I created two Scenes in flash (start scene and Main scene) and I have a class called Cloud.as..The cloud movements work fine in the start scene but once I hit the start button to go in the next scene, I am having an error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Cloud/wrapCloud()[Project\Jack_Crawford_System\Cloud.as:46]
at Cloud/onEnter()[Project\Jack_Crawford_System\Cloud.as:37]

Can anyone please tell me what should I do? There is no code in the Main scene

Here is the code for the Cloud.as:

package
{
    import flash.display.MovieClip;
    import flash.events.Event;

    public class Cloud extends MovieClip
    {
        private var _cloudpixel:Number;
        private var _halfCloudWidth:Number;

        public function Cloud()
        {
            addEventListener(Event.ADDED_TO_STAGE, onAddedToStage);
        }

        private function onAddedToStage(event:Event):void
        {
            _cloudpixel = -1;
            _halfCloudWidth = this.width/2;

            addEventListener(Event.ENTER_FRAME, onEnter);
            addEventListener(Event.REMOVED_FROM_STAGE, onRemovedFromStage);
            removeEventListener(Event.ADDED_TO_STAGE, onAddedToStage);
        }
        private function onRemovedFromStage(event:Event): void
        {
            removeEventListener(Event.REMOVED_FROM_STAGE, onRemovedFromStage);
        }
        private function onEnter(event:Event):void
        {
            this.x +=_cloudpixel;
            wrapCloud();
        }
        private function wrapCloud():void
        {
            if (this.x + _halfCloudWidth<0)
            {
                this.x = stage.stageWidth + _halfCloudWidth;
            }
        }
    }
}

Thank you!