Navigation button not adding in window

I am trying to load window which has web view but i also want the back button. I tried something but its not working. here is the code In XLM <NavigationWindow id="navWin" platform="iOS"> In Cotroller Alloy.Globals.navGroup = $.navWin;

Code: var win = Ti.UI.createWindow({ title:'Check From MSDS', backButtonTitle: 'Back'

        });

            var webview = Ti.UI.createWebView({ 
            url:encoded_url
        });

            win.add(webview);
            Alloy.Globals.navGroup.openWindow(win);

Can anyone help?