Can not alert or popup in module

Question

I am pretty much new to modular applications so here's my problem.
I wanna show up a popup window in the module, and I need the window to be modular
I don't know how to work around this. Do I need popup everything from main application?
main application code
<?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" minWidth="955" minHeight="600">
<fx:Declarations>
<!-- Place non-visual elements (e.g., services, value objects) here -->
</fx:Declarations>
<s:ModuleLoader url="TestAlert.swf">
</s:ModuleLoader>
</s:Application>
module code
<?xml version="1.0" encoding="utf-8"?>
<s:Module 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="400" height="300">
<fx:Script>
<![CDATA[
import mx.controls.Alert;
protected function button1_clickHandler(event:MouseEvent):void
// TODO Auto-generated method stub
Alert.show("hmmmm...","something wrong");//error here
]]>
</fx:Script>
<fx:Declarations>
<!-- Place non-visual elements (e.g., services, value objects) here -->
</fx:Declarations>
<s:Button x="176" y="119" label="Button" click="button1_clickHandler(event)"/>
</s:Module>

Answer 1

the error I got is:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
     at mx.managers::PopUpManagerImpl/http://www.adobe.com/2006/flex/mx/internal::createModalWindow()[E:\dev\hero_private\framew orks\projects\framework\src\mx\managers\PopUpManagerImpl.as:701]
     at mx.managers::PopUpManagerImpl/addPopUp()[E:\dev\hero_private\frameworks\projects\framewor k\src\mx\managers\PopUpManagerImpl.as:400]
     at mx.managers::PopUpManager$/addPopUp()[E:\dev\hero_private\frameworks\projects\framework\s rc\mx\managers\PopUpManager.as:193]
     at mx.controls::Alert$/show()[E:\dev\hero_private\frameworks\projects\mx\src\mx\controls\Ale rt.as:618]
     at TestAlert/button1_clickHandler()[/Users/dawn/Documents/workspace/work/dev/sandbox/modulet est/src/TestAlert.mxml:12]
     at TestAlert/___TestAlert_Button1_click()[/Users/dawn/Documents/workspace/work/dev/sandbox/m oduletest/src/TestAlert.mxml:20]