logic/flow aside, still the error is on engine side - validating the VdsNetworkInterface entity which
seems to be missing an "interfaces" field or method thus throwing this runtime error.
I wonder if GWT serialized something unexpected here.
Alan - can you try invoking setup networks and see if any other action is terminated the same way?

Yes, no matter what I do on Setup Host Networks, I get the same error when I click OK. Even if I make not changes at all, I get the same when I click OK. I have to Cancel to get out of that screen.