Hi I'm new to GitHub, don't speak English well and NULL in Java.

1.16.5-3.5

There is a bug when this mod works with the EnderMail and SnailMail mods.
I looked at the logs, your mod does "ent.serializeNBT()" and this causes the "Saving entity NBT" to crash when creating a mob from these mods.
I tried commenting out the loop with actions on "pet", thereby breaking the possibilities with pets, but that removed the crashes.
I am 40% sure that this check can break other interactions from mods.

Line with checking

• Music-Triggers/src/main/java/mods/thecomputerizer/musictriggers/client/MusicPicker.java

  Line 317 in 9ca998c

  if (ent.serializeNBT().getString("Owner").matches(player.getName())) {

Logs:
2021-12-12-6.log

Fixed in V4.0

Right, I forgot to respond to this, but it should be fixed with the update tomorrow. Thanks for reporting