URGENT-- Badly stuck to find the Unique WIndow Id of a PopUp

Hi,
Please help me, I am badly stuck to found the Unique Window id of a Pop Up under a POP up :-

[b:1de1zlvw]NOTE :- Please refer the attached sheet in which i describe step by step and look into below complete code :-[/b:1de1zlvw]
[b:1de1zlvw]
COMPLETE CODE :-[/b:1de1zlvw]


import java.sql.Driver;
import java.util.Iterator;
import java.util.Set;
import java.util.concurrent.TimeUnit;

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
//import org.openqa.selenium.firefox.FirefoxDriver;

public class MMs_test {
static WebDriver driver;
public static void main(String[] args) throws InterruptedException {
System.setProperty("webdriver.chrome.driver","D:\\chromedriver.exe");
driver = new ChromeDriver();
driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
driver.get("http://172.19.4.64/MR29/MMS/index.aspx");
Set<String> mainwindow = driver.getWindowHandles();
System.out.println("Total Numbers of the page --- > " +mainwindow.size());
Iterator<String> itr = mainwindow.iterator();
String FirstWindow = itr.next();
String SecondWindow = itr.next();
System.out.println("Parent Window id is --- >" +FirstWindow);
System.out.println("Child Window id is --- >" +SecondWindow);
driver.switchTo().window(SecondWindow);
driver.findElement(By.xpath("//*[@id='txtUserName']")).sendKeys("careteststh");
driver.findElement(By.xpath("//*[@id='txtPassword']")).sendKeys("irctc@116");
driver.findElement(By.xpath("//*[@id='btnLogin']")).click();
//Thread.sleep(1000);
driver.findElement(By.xpath("//*[@id='5001']/div/span")).click();
driver.findElement(By.xpath("//*[@id='5002']/div/span")).click();
driver.findElement(By.xpath("//*[@id='5003']/div/span")).click(); -----> [b:1de1zlvw]Code is working fine till that line but on next line i am not able to find out the unique window id of a pop up[/b:1de1zlvw]
mainwindow = driver.getWindowHandles();
itr = mainwindow.iterator();
itr = mainwindow.iterator();
String FirstWindow1 = itr.next();
String SecondWindow1 = itr.next();
System.out.println("Parent Window id is --- >" +FirstWindow1);
System.out.println("Child Window id is --- >" +SecondWindow1);
driver.switchTo().window(SecondWindow1);
System.out.println(driver.getTitle());
}

}