are you running debconf as root?
I am not sure. I just follow the guide:
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#chrooting inside d-i/build/tmp/gtk-miniiso/tree/
sudo chroot ./ sh
#creating all device files needed to access the framebuffer
cd /dev
mknod fb0 c 29 0
mknod tty c 5 0
mknod tty0 c 4 0
mknod tty8 c 4 8
#starting the debian-installer
debconf-loadtemplate d-i /var/lib/dpkg/info/*.templates
export DEBIAN_FRONTEND=gtk
debconf -f gtk -o d-i /usr/bin/main-menu
---8< ----------
After that, system crash. Keyboard and mouse won't respond.