[flexcoders] Problem with viewstack in Flex 2.0 ActionScript not search element by id.

[j_hora]

Hallo,

i start with Flex2 and have this problem.
ActionScript not search Label by id in the second Canvas in ViewStack.
Application start with error TypeError: Error #1009: Cannot access a
property or method of a null object reference.
Id "labela" search OK, but "labelb" is for the function "invisible".
What's wrong? 
My application:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml";
layout="absolute"  creationComplete="start();">

<mx:Script>
        <![CDATA[
          
           public function start():void
           {
           labela.text = "new text A";
           labelb.text = "new text B";
           }
        ]]>
    </mx:Script>
   
    <mx:TabBar   height="30" width="200" id="tab_menu"  
dataProvider="stack1" >
        </mx:TabBar>
    <mx:ViewStack x="0" y="40" width="250" height="100"
borderStyle="solid" id="stack1">
    <mx:Canvas label="aaa">
            <mx:Label width="200" id="labela" text="old text A"/>
    </mx:Canvas>        
    <mx:Canvas label="bbb">
            <mx:Label width="200" id="labelb" text="old text B"/>
    </mx:Canvas>        

    </mx:ViewStack>
        
</mx:Application>



Very thanks for help

Jan Hora