I just tried to spawn them again through the F6 menu and got the object reference error. I removed all mods and did a fresh copy of enZombies to remove any chance of me meddling around and messing something up. You said the items.xml file, I can see the boss hand items in the entityclasses file, but only the businessman and scientist boss hands are in my items file. I have version 2.7 of the mod and version a20.6(b9) of the game. I'm assuming the hand items are supposed to be in the items file and it's throwing errors at me because the zombies are trying to spawn in with non-existent items?
I just went to the main page and saw that version 2.71 is out. I downloaded that, verified that the hand items are in the items file, and I can spawn in the bosses now. Thanks for pointing me in the right direction to figure that out.
On an unrelated note, is there a good discord around for general modder questions? I'm interested in doing a bit of modding and I'm sure I'll run into some issues.