org.openqa.selenium.NoSuchWindowException: Window not found. The browser window may have been closed while closing the window through driver i.e driver.close()
The driver has been switched to a non existent window or the window has already been closed.
String subWindowHandler = null;
Set<String> handles = driver.getWindowHandles();
Iterator<String> iterator = handles.iterator();
subWindowHandler = iterator.next();
driver.switchTo().window(subWindowHandler); // switch to the child window
driver.findElement(By.id("closeChildWindowButton")).click(); // You already closed the child window
driver.close(); // this will throw the exception as the child window has already been closed in the previous code.