nanogui: Thread: passing window id to different handler in tinywidget

Hi

In my tiny widget application when I press new file button it invokes new file handler and then open a new window & file there user can write something on it 

When I press save file button. It invokes save file handler. But I don't know which window id I have to pass it, so that I can save the image. 


Regards
Puspraj