TypeError: Error #1009: 无法访问空对象引用的属性或方法

TypeError: Error #1009: 无法访问空对象引用的属性或方法。
at FirstMap/search_clickHandler()[E:\Flash Builder\Map\src\FirstMap.mxml:17]
at FirstMap/__search_click()[E:\Flash Builder\Map\src\FirstMap.mxml:30]

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx"
width="100%" height="100%" xmlns:chart="chart.*">
<s:layout>
<s:BasicLayout/>
</s:layout>

<fx:Script>
<![CDATA[
import mx.controls.Alert;

protected function search_clickHandler(event:MouseEvent):void
{
var teacher:Teacher = new Teacher();
var workId:String = teacher.workId.label;
Alert.show(workId);
}
]]>
</fx:Script>

<fx:Declarations>
<!-- 将非可视元素（例如服务、值对象）放在此处 -->
</fx:Declarations>

<mx:HBox width="100%" height="100%" paddingBottom="10" paddingLeft="10" paddingRight="20"
paddingTop="10">
<s:Panel width="50%" height="100%">
<s:Button id="search" label="查询" click="search_clickHandler(event)" width="100%"
height="100%" fontSize="320"/>
</s:Panel>
<chart:Teacher width="50%" height="100%"/>
</mx:HBox>

</s:Application>