When an angered Warden is serialized, "Not a string" appears as an error message in the chat.
- Use spawn eggs to spawn a Warden and a couple of Zoglins.
- When they start to fight, trigger the serialization of Warden by saving the world, pausing the game, or using the /data get command
- -> "Not a string" appears in the log. If you use the /data get command, you will find that there's no anger field in the data.
- When all the Zoglins are killed, trigger the serialization again
- -> The message does not appear. If you use the /data get command, you will find that the anger field is correctly serialized.
Code analysis by SPGoding can be found in this comment.