overlay-scrollbar crashes on GtkOffscreenWindow

In my application i'm use GtkOffscreenWindow as parent window when hide main window.
Crash happens when overlay-scrollbar is child of GtkOffscreenWindow,
and user change dconf settings com.canonical.desktop.interface.scrollbar-mode

To reproduce:
install my program from ppa https://launchpad.net/~linvinus/+archive/altyo
run altyo from gnome-terminal
hide it (alt+<tilda> by default)
change setting in dconf com.canonical.desktop.interface.scrollbar-mode
to normal and then back to overlay-auto
result: program will crash

my application sources available there https://github.com/linvinus/AltYo
ubuntu 12.10 x86_64
overlay-scrollbar-gtk3:amd64 0.2.16+r357-0ubuntu1

Probably overlay scrollbar should check is priv->tail_window and priv->bar_window mapped, before create new gdk window.