I disabled all other mods and only had this mod on. Exact same issue, but slightly different.
zombieGeist
Then NRE Error.
Then zombieCheerLeaderRadiated spawns in next.
Sometimes the FatCopt spawns in after the NRE error now, as previously with my other mods it was always zombieYo that spawned every time.
What could the issue be? Edit: I believe it to be the zombieGeist. When I try to spawn in X amounts of them, the game throws an NRE at me. Edit2: Testing it more, I believe it's actually because too many Zombies are being spawned in general.
Edit3: However, the zombieGeist are the only issue when you hold down Shift and click to spawn them in the entity spawner with F6.