Fixed in git Head.
With these two commits.
GtkD didn't check fot null when creating Objects:
https://github.com/gtkd-developers/GtkD/commit/c99e2ff228fea744297a2838032831cb1d1aa8da
A small typo in the CustomList implementation:
https://github.com/gtkd-developers/GtkD/commit/169f1583c35fdc1c13f8768acb777955b2a44f82