I want to load external swf but it’s give me error

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Main()[E:\MacMenuSource\Main.as:23]

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,buttonCl ick);

mc_gear.addEventListener ( MouseEvent.CLICK , buttonClick );

mc_apple.addEventListener(MouseEvent.CLICK,buttonC lick);

mc_photo.addEventListener(MouseEvent.CLICK,buttonC lick);

mc_buy.addEventListener(MouseEvent.CLICK,buttonCli ck);

mc_contact.addEventListener(MouseEvent.CLICK,butto nClick);

//_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 = “bg2.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);

}

}

}

the line 23 is

mc_star.addEventListener(MouseEvent.CLICK,buttonCl ick);

if you want to see the fla download it from this link
http://mir.cr/0BJTHHF4

thank you