Flashbuilder 4.5 Alert.show error

My shutdown method looks like

    private function onShutDown(event:Event):void {
            event.preventDefault();
            Alert.show("Do you REALLY want to close this application?", "Wait! Don't go!", 3, null, onConfirmation);
        }

And is valid but throws :

[SWF] AirPureAS.swf - 985,029 bytes after decompression
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at mx.controls::Alert$/show()[E:\dev\hero_private\frameworks\projects\mx\src\mx\controls\Ale rt.as:566]
    at AirPureAS/onShutDown()[C:\Users\Public\Documents\workspace\study\AirPureAS\src\AirPureAS. as:64]

Am I missing something. I have this setup so that the Flex project runs from a default .as class rather then default mxml application class.

And it works fine. As long as I leave Alert.show alone...

My hair is gone, my tea is cold, heeeeeelp!

Thanks on beforehand

Thomas