Simple example of webview in appcelerator

gistfile1.txt

// this sets the background color of the master UIView (when there are no windows/tab groups on it)
Titanium.UI.setBackgroundColor('#000');

// create tab group
var tabGroup = Titanium.UI.createTabGroup();


//
// create base UI tab and root window
//
var win1 = Titanium.UI.createWindow({  
    title:'Tab 1',
    backgroundColor:'#fff'
});
var tab1 = Titanium.UI.createTab({  
    icon:'KS_nav_views.png',
    title:'Tab 1',
    window:win1
});

var tweetButton = Ti.UI.createButton({
	title:'Tweet',
	height:35,
	left:10,
	right:10
});

win1.add(tweetButton);

tweetButton.addEventListener('click', function(data) {
var tweetWindow = Titanium.UI.createWindow();

var webview = Titanium.UI.createWebView({
		url:'http://www.google.com'
	}); 
	tweetWindow.add(webview);
	tweetWindow.open({
		modal:true
	});

	var closeTweetButton = Titanium.UI.createButton({
		title:'Close'
	});
	closeTweetButton.addEventListener('click', function() {
		tweetWindow.close();
	});
	tweetWindow.setLeftNavButton(closeTweetButton);
});


//
//  add tabs
//
tabGroup.addTab(tab1);


// open tab group
tabGroup.open();

I don't see any close button when I run this code