Hi,
you can’t stop the flow in a Vaadin application. Vaadin is based on servlet technology and this kind of approach just can’t work there.
Instead, you should implement e.g. a some kind of callback listener class which you hand over to your window class when creating the window. Then on window close, you should call the callback method with the selected customer as a parameter. The implementation of this callback method could then do what ever you needed to do afterwards.
-tepi