1.21.1
Minecraft ...
Portable Mobs ...

Put any mob in master capture cell on a server
...

Error: java.lang.NullPointerException: Cannot invoke "net.minecraft.server.MinecraftServer.method_30611()" because the return value of "net.minecraft.class_1496.method_5682()" is null
...

I tried to replicate the issue, but it works fine for me? Can you tell me the modloader you used (Forge, Fabric or NeoForge)?