"Not a string" appears in log when serializing angered Warden

When an angered Warden is serialized, "Not a string" appears as an error message in the chat.

To Reproduce

1. Use spawn eggs to spawn a Warden and a couple of Zoglins.
2. 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.
3. 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

Code analysis by [Mod] SPGoding can be found in this comment.