I’ve been through that tutorial. One note I have related to Get Default Item Object is that the tutorial says to make it private but a child can’t override if it’s private (at least in 5.1.1) (so it should not be private).
That could be the issue?
Otherwise there’s any number of places it could go wrong, so I’d start tracking it down with print strings, to find where data is first missing, starting in the item when it gets picked up. Can it print the class… if not… it’s somewhere before that.