BackSpace causes Apps to Exit

Smartgwt version-SmartGwt 3.0
Browser version-mozilla firefox 25,chrome,IE 10

Window.addWindowClosingHandler(new ClosingHandler() {
@Override
public void onWindowClosing(ClosingEvent event) {
event.setMessage("");
}
});

The above listener works fine for backspace button or reloading page but the issue i am facing is like whenever I am downloading any document in application is again
asking the same message.So I need to avoid this in case of downloading docs.Can anyone suggest me how to solve this issue?