Example about how to handle window closing in Android, doing: Cleaning of the window's contents, remove all listeners and proper closing and null of the window.

app.js

Ti.include('utils.js');

// Base window
var win = Ti.UI.createWindow();

// Base launch button
var button = Ti.UI.createButton({
  title:'Press here',
	top: 10,
	left: 10,
});

// Function to clean the win2 (i.e., remove all the attached listeners, close it, null it)
function win2Clean(){
	if(win2!=null){
		win2.close();
		clean(win2);
		unRegisterAllEventListeners(win2);
		win2 = null;
	}	
	Ti.App.removeEventListener('win2:close',win2Clean);
}

var win2 = Ti.UI.createWindow({
	backgroundColor:'white',
});

function win2Close(){
	Ti.App.fireEvent('win2:close');
}

button.addEventListener('click',function(){
	win2 = win2 || Ti.UI.createWindow({
		backgroundColor:'white',
	});
	registerEventListener(win2, {event: 'android:back', callback: win2Close});
	Ti.App.addEventListener('win2:close',win2Clean);
	win2.open();
});

win.add(button);
win.open();

utils.js

/// Recursive Clean of Memory 
// by Mauro Parra
// https://gist.github.com/mauropm/2655813
// Begin recursive clean of memory
function do_clean(e,c){
  clean(c);
	e.remove(c);
	Ti.API.info( 'Deleted child at do_clean' );
	return;
}
 
function clean(e){
	if (e!=null){
		if(e.children){
			Ti.API.info( 'Number of children: ' + e.children.length );
			for(var i = 0; i<e.children.length;i++){
				do_clean(e, e.children[0]);
			}
		} else { 	
			return; 
		}
	}
}

// End recursive clean of memory

/// <<< Register & UnRegister Event Listeners
// Original from: https://gist.github.com/minhnc/2333095
// Thanks minhnc! 

 
/**
 * params: {event: 'event', callback: eventCallback}
 */
function registerEventListener(obj, params) {
	if ( typeof obj._eventListeners == 'undefined' ) {
		obj._eventListeners = [];	
	}
	
	obj.addEventListener(params.event, params.callback);
	
	var eventListeners = obj._eventListeners;
	eventListeners.push(params);
	obj._eventListeners = eventListeners;
	
	Ti.API.info( JSON.stringify(obj._eventListeners) );
}
function unRegisterAllEventListeners(obj) {
	if ( typeof obj._eventListeners == 'undefined' || obj._eventListeners.length == 0 ) {
		return;	
	}
	
	for(var i = 0, len = obj._eventListeners.length; i < len; i++) {
		var e = obj._eventListeners[i];
		obj.removeEventListener(e.event, e.callback);
	}
 
	obj._eventListeners = [];
}
 
/// Register & UnRegister Event Listeners >>>