so this:
notifyOnRender2D(function(g:kha.graphics2.Graphics) {
g.begin();
g.color = Color.Yellow;
g.drawRect(20,20,20,20,1);
g.end();
});
now gives this:
#
# Fatal error in v8::ToLocalChecked
# Empty MaybeLocal.
#
EDIT: g.begin() and g.end() need to be used in a different way here.