Cancel WWW.LoadFromCacheOrDownload asset bundle

I'm try use the WWW.LoadFromCacheOrDownload function thru
http://docs.unity3d.com/Manual/keepingtrackofloadedassetbundles.html
for avoid the error of

ok, it's work!
But i got a problem,
how to cancel the www when i wanna cancel it?
I'm try to write www.Dispose();
but the apps will crash,
any help?

 

Found this thread:
http://forum.unity3d.com/threads/how-to-stop-the-www-request.36065/

Try to use Dispose() on your wwwObject. Not on the www class www.Dispose().

 

WWW is wwwObject
Just still same, will crash the apps,
any help?