Checking browser version form Flex

[scott matheson]

Hi
    I tried some code  to check the browser versions, but i get back a NULL

userAgent = ExternalInterface.call("window.navigator.userAgent.toString");


following this 
https://developer.mozilla.org/en-US/docs/Web/API/Window/navigator 
<https://developer.mozilla.org/en-US/docs/Web/API/Window/navigator>

any one point me in the right direction 

Scott