[plt-scheme] Problem for asking "are you sure?" before closing a frame

Hi!
I have written
(define diagram-frame%
  (class frame% ()
     (define/augment (on-close)
         (send this show #t)
         (send QUIT-FRAME show #t))
  (super-new)))

QUIT-FRAME is a frame which ask "are you sure you want to qui?" with
buttons "yes" and "no", and after "Do you like to save your document?"
My problem is that my diagram-frame disappear when I click on the
window close button at the top, in the same time that my QUIT-FRAME
appear. I have tried (send this show #t) like you can see but it
doesn't work. I would like to say at my diagram-frame "wait the answer
before closing or not!!!!".
Could you help me please?
Thank you for your answers.