Can't access property of null object

hey guys. I need your help please.
i am pretty sure i am doing something wrong. just donno what it is.

i have frame HOME.
above this frame i have a code for the buttons more003 and more004.

more004 is on frame called HUMAN.

when i preview is get the error 1009: Can’t access…

here is the AS:

[COLOR=“Red”]more003.addEventListener(MouseEvent.CLICK,klik3);
more004.addEventListener(MouseEvent.CLICK,klik4);

function klik3(event:MouseEvent):void {
makePopUp3();
};
function klik4(event:MouseEvent):void {
makePopUp4();
};

function makePopUp3() {

var popupMc:PopUp = new PopUp();
popupMc.x = 10;
popupMc.y = 10;
popupMc.name = "popup";
TransitionManager.start(popupMc,{type:Fade, direction:Transition.IN, duration:0.2,  easing:Strong.easeIn});
addChild(popupMc);
loadTxt3();
popupMc.popClose.addEventListener(MouseEvent.CLICK,Close);

};
function makePopUp4() {

var popupMc:PopUp = new PopUp();
popupMc.x = 10;
popupMc.y = 10;
popupMc.name = "popup";
TransitionManager.start(popupMc,{type:Fade, direction:Transition.IN, duration:0.2,  easing:Strong.easeIn});
addChild(popupMc);
loadTxt4();
popupMc.popClose.addEventListener(MouseEvent.CLICK,Close);

};
///
function Close(e:Event) {
removeChild(e.currentTarget.parent);
};

function loadTxt3() {

var fileAdres:URLRequest = new URLRequest("Txt/Home/Students.txt");

var loader:URLLoader = new URLLoader(fileAdres);

fileAdres.method=URLRequestMethod.POST;
loader.dataFormat=URLLoaderDataFormat.TEXT;

loader.addEventListener(Event.COMPLETE,textLoaded);

};
function loadTxt4() {

var fileAdres:URLRequest = new URLRequest("Txt/Human/Course.txt");

var loader:URLLoader = new URLLoader(fileAdres);

fileAdres.method=URLRequestMethod.POST;
loader.dataFormat=URLLoaderDataFormat.TEXT;

loader.addEventListener(Event.COMPLETE,textLoaded);

};
//
function textLoaded(event:Event):void {

var loader:URLLoader = URLLoader(event.target);

var txt:String = loader.data;

PopUp(getChildByName("popup")).field.htmlText=loader.data;

};[/COLOR]


hope u can understand.

can anyone help me please?..