Crash log here!
Version: 1.1.2
Using 'Mob Disguises' mod
Minecraft 1.18.1

I was on a singleplayer world, and had run /disguise @e as minecraft:player to disguise every entity as a player, and proceeded to punch a nearby cat (now disguised as a player) to death. The server crashed with an NPE when 'Ticking player', unable to get the networkHandler field in ServerPlayerEntity.

This also caused the world to report the same error anytime an attempt to load it up again was made, rendering it unplayable.

I understand that 1.1.2 is not the latest DisguiseLib, but it is the latest that works with the 'Mob Disguises' mod, and thus is what I used. I am unsure if the issue is reproducible on 1.2.0.

It's probably reproducible, as there weren't many changes in 1.2.0

could be that network handler is trying to send "you died" packet to diguised entity ... I'll have to investigate

Ok, this is really weird, as it crashes when you pick up a new item therefore unlock new recipes.
I could reproduce it with first cow I killed, however, afterwards none of the cows caused the crash anymore.
Same happened with polar bears.

No baby animals were hurt while debugging.

Further observations; it seems that crash can occur with other entities as well, but only if you pick up the dropped items immediately after their death 🤔