How to fix this error

i want load external swf it's give me error

 

typeerror: error #1009: cannot access property or method of null object reference.

    @ main()

 

the code is

 

 

package{

   

    import flash.display.sprite;

    import flash.events.mouseevent;

    import flash.net.navigatetourl;

    import flash.net.urlrequest;

    import flash.display.loader;

    public class main extends sprite{

       

        private var _loader:loader = new loader();

       

        public function main():void{

            mc_star.addeventlistener(mouseevent.click,buttonclick);

            mc_gear.addeventlistener(mouseevent.click,buttonclick);

            mc_apple.addeventlistener(mouseevent.click,buttonclick);

            mc_photo.addeventlistener(mouseevent.click,buttonclick);

            mc_buy.addeventlistener(mouseevent.click,buttonclick);

            mc_contact.addeventlistener(mouseevent.click,buttonclick);

            _loader=new loader();

            addchild(_loader);

        }

       

        private function buttonclick(e:mouseevent):void{

            var url:urlrequest=new urlrequest();

            switch(e.target.name){

                case "mc_star": url.url="bg2.swf"; break;

                case "mc_gear": url.url="bg3.swf"; break;

                case "mc_apple": url.url="bg4.swf"; break;

                case "mc_photo": url.url="bg5.swf"; break;

                case "mc_buy": url.url="bg6.swf"; break;

                case "mc_contact": url.url="bg7.swf"; break;

            }

            _loader.load(url);

        }

    }   

}

use:

 

package{

   

    import flash.display.sprite;

    import flash.events.mouseevent;

    import flash.net.navigatetourl;

    import flash.net.urlrequest;

    import flash.display.loader;

    public class main extends sprite{

       

        private var _loader:loader = new loader();

       

        public function main():void{

            mc_star.addeventlistener(mouseevent.click,buttonclick);

            mc_gear.addeventlistener(mouseevent.click,buttonclick);

            mc_apple.addeventlistener(mouseevent.click,buttonclick);

            mc_photo.addeventlistener(mouseevent.click,buttonclick);

            mc_buy.addeventlistener(mouseevent.click,buttonclick);

            mc_contact.addeventlistener(mouseevent.click,buttonclick);

            //_loader=new loader();

            addchild(_loader);

        }

       

        private function buttonclick(e:mouseevent):void{

            var url:urlrequest=new urlrequest();

            switch(e.target.name){

                case "mc_star": url.url="bg2.swf"; break;

                case "mc_gear": url.url="bg3.swf"; break;

                case "mc_apple": url.url="bg4.swf"; break;

                case "mc_photo": url.url="bg5.swf"; break;

                case "mc_buy": url.url="bg6.swf"; break;

                case "mc_contact": url.url="bg7.swf"; break;

            }

            _loader.load(url);

        }

    }   

}

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