buttons.rb example crashes on exit

This happens also under Debian Squeeze running ruby 1.9.2-p180 and fxruby-1.6.20.

It seems that the destruction of the application is not working correctly, because
the following change - a bottom-up removal of all children before exiting - fixes
the crash:

--- button.rb   2011-04-21 10:05:17.612782000 +0200
+++ button.rb.fixed 2011-04-21 10:07:27.540782516 +0200
@@ -129,7 +129,14 @@
     FXRadioButton.new(group4, "Apart",  @group4_dt, FXDataTarget::ID_OPTION + 3)
 
     quitButton = FXButton.new(controls, "&Quit", :opts => FRAME_RAISED|FRAME_THICK|LAYOUT_FILL_X)
-    quitButton.connect(SEL_COMMAND) { getApp().exit(0) }
+    quitButton.connect(SEL_COMMAND) { clean_up(self); getApp().exit(0) }
+  end
+
+  def clean_up(component)
+    component.children.each do |child|
+      clean_up(child)
+      component.removeChild(child)
+    end
   end
 
   # Load the named icon from a file

This is solved in 1.6.22.