Why have error "Access of undefined property"?

In this class I have error about two variables myFormat and AboutText. Can't understand why? There is import line, and class in its own file. Before this error all text of this class were in main.as and working fine. When I tried to move code in different class the error ocures.

package 
{
     import flash.display.Sprite;
     import flash.events.Event;
     import flash.display.*;
     import flash.events.MouseEvent;
     import flash.system.fscommand;
     import flash.text.*;
 
     public class MenuScreen extends Sprite 
     {
          var myFormat:TextFormat = new TextFormat();     
          var AboutText:TextField = new TextField();
 
          var btn_play:MyButtonClass = new MyButtonClass();
          var btn_opt:MyButtonClass = new MyButtonClass();
          var btn_inst:MyButtonClass = new MyButtonClass();
          var btn_abt:MyButtonClass = new MyButtonClass();
          var btn_ext:MyButtonClass = new MyButtonClass();
 
 
          myFormat.font = "Times New Roman";
          myFormat.size = 20;
           myFormat.color = 0x000000;                    
 
          AboutText.width = 550;
          AboutText.height = 250;
          AboutText.x = 225;
          AboutText.y = 80;
          AboutText.selectable = false;
          AboutText.setTextFormat(myFormat);
          addChild(AboutText);
 
          public function MenuScreen() 
                    {
                    super();
 
 
                         btn_play.name = "btn_PLAY";
                         btn_play.label = "Play";
                         btn_play.addEventListener(MouseEvent.CLICK, btnMouseHandle);
                         addChild(btn_play).y = 50;
                         btn_play.x = 80
 
 
                         btn_opt.name = "btn_OPTIONS";
                         btn_opt.label = "Options"
                         btn_opt.addEventListener(MouseEvent.CLICK, btnMouseHandle);
                         addChild(btn_opt).y = 100;
                         btn_opt.x = 80
 
 
                         btn_inst.name = "btn_INSTRUCTIONS";
                         btn_inst.label = "Instructions"
                         btn_inst.addEventListener(MouseEvent.CLICK, btnMouseHandle);
                         addChild(btn_inst).y = 150;
                         btn_inst.x = 80
 
 
                         btn_abt.name = "btn_ABOUT";
                         btn_abt.label = "About"
                         btn_abt.addEventListener(MouseEvent.CLICK, btnMouseHandle);
                         addChild(btn_abt).y = 200;
                         btn_abt.x = 80
 
 
                         btn_ext.name = "btn_EXIT";
                         btn_ext.label = "Exit"
                         btn_ext.addEventListener(MouseEvent.CLICK, btnMouseHandle);
                         addChild(btn_ext).y = 250;
                         btn_ext.x = 80
                    }
 
          function btnMouseHandle(e:MouseEvent):void
                    {                         
                         switch(e.target.name){
                              case "btn_PLAY":
                                   var TestSWC:MovieClip = new WSymbol_mc;
                                   trace("btn_01 click");
                                   TestSWC.x = 50;
                                   TestSWC.y = 50;
                                   addChild(TestSWC);
                                   break;
                              case "btn_OPTIONS":
                                   trace("btn_02 click");
                                   break;
                              case "btn_INSTRUCTIONS":
                                   trace("btn_03 click");
                                   ShowTextBlock (e.target.name);
                                   break;
                              case "btn_ABOUT":
                                   trace("btn_04 click");
                                   ShowTextBlock (e.target.name);
                                   break;
                              case "btn_EXIT":
                                   trace("btn_05 click");
                                   fscommand('quit');
                                   break;
                         }
                    }
 
          function ShowTextBlock (strID:String):void
                    {          
                         if (strID == "btn_INSTRUCTIONS")
                              {
                                   trace("INS");
                                   AboutText.text = "Some text here";
                              }
                         else 
                              {
                                   trace("AB");
                                   myFormat.align = TextFormatAlign.CENTER;
                                   myFormat.leading = 10;
                                   AboutText.defaultTextFormat = myFormat;
                                   AboutText.text = "Some text here";
                              }
                    }
 
     }
 
}

This is the main.as:

package
{
     import flash.display.Sprite;
     import flash.events.Event;
     import flash.display.*;
     import flash.events.MouseEvent;
     import flash.system.fscommand;
     import flash.text.*;
 
     public class Main extends Sprite
     {
 
          var MScreen:MenuScreen = new MenuScreen;
 
 
 
          public function Main():void
          {
               if (stage) init();
               else addEventListener(Event.ADDED_TO_STAGE, init);
          }
 
          private function init(e:Event = null):void
          {
               removeEventListener(Event.ADDED_TO_STAGE, init);
               addChild(MScreen);
 
 
          }
 
     }
 
}

One man told me that the reason in too many actions outside functions, is it true? Is it restriction in AS3?