[mikhailk]

этот код рабочий
я закомментил необъявленные bodyBox и testViewer и все отработало:

internal var contentViewer:TextField = new TextField();
internal var contentFormat:TextFormat = new TextFormat();
internal var urlReqMain:URLRequest = new URLRequest('test.txt');
internal var urlLoadMain:URLLoader = new URLLoader();
internal var mainText:String;
//contentViewer.x = bodyBox.x + 15;
//contentViewer.y = bodyBox.y + 15;
contentViewer.height = 810;
contentViewer.width = 738;
contentViewer.border = true;
contentFormat.size = 20;
contentViewer.condenseWhite=true;
contentViewer.multiline = true;
contentFormat.font = 'Calibri';
contentFormat.leading = -3;
contentViewer.wordWrap = true;
contentViewer.autoSize = TextFieldAutoSize.CENTER;
addChild(contentViewer);
//.................................
urlLoadMain.load(urlReqMain);
urlLoadMain.addEventListener(Event.COMPLETE, urlLoadMainComplete);
 
button1.addEventListener(MouseEvent.CLICK,button1Clicked);
//.
//.
//.
function urlLoadMainComplete(event:Event):void {
	mainText = urlLoadMain.data;
}
function button1Clicked(event:Event):void {
    contentViewer.htmlText = mainText;
	//testViewer.htmlText = contentViewer.htmlText;
 
}

ищите ошибку в другом