When pressing right-click while holding a "Message in a bottle" the item gets consumed and the error message "Nullpointer exception: Could not convert null string" or something like that.

Minecraft version: 1.21.1
NeoForge Version: 21.1.113
Aquaculture 2 version: 1.21.1. - 2.7.13
in Modpack ATM 10 - All the mods

The "Message in a bottle"-item can show a multitude of different message, when you get one an right-click it.
The "Nullpointer exception: Could not convert null string"-message is just one of them. It's a joke - nothing actually went wrong :)