I have tried it, it works for you.
String mainWinHander = webDriver.getWindowHandle();
// code for clicking button to open new window is ommited
//Now the window opened. So here reture the handle with size = 2
Set<String> handles = webDriver.getWindowHandles();
for(String handle : handles)
{
if(!mainWinHander.equals(handle))
{
// Here will block for ever. No exception and timeout!
WebDriver popup = webDriver.switchTo().window(handle);
// do something with popup
popup.close();
}
}