NPE when player being attacked by boss

Summary

As the title. See below for repos and logs.

Steps to reproduce

When the boss (Alpha_spider) attacks a player, an exception will be thrown on the console

1. Spawn the boss: id: Alpha_spider
2. Let the boss attack players
3. Each attack generates a NPE on the console

My combined MM config files: https://pastebin.com/fjDQ3qaJ

Current behavior

There will be a NPE if the boss attacks players.

Intended correct behavior

There should not be NPE when the boss attacks players.

Server log file

https://pastebin.com/Z5JhxgwT (contains the logs of NPEs)

Debug log snippet

https://pastebin.com/Z5JhxgwT (the same as the server log file)

Proposed fixes

I have no idea, sorry.