Exception when dragon killed. 1.20.4

[NeverGrowUp]

Description of bug:
Spawning of dragon in the end works fine, but upon killing, an exception occurs. Same exception occurs on each dragon kill thereafter as well. Dragon fill animation, egg drop, XP drop all works fine after the kill.

Steps to reproduce the behavior:
Default configs were used throughout with the exception of additional blueprints.
So dragon didn't spawn on entry to the end as per default config, so end_crystal had to be placed to create exit portal, then 4 end_crystals on portal to spawn dragon.
Spawn dragon, then kill. Upon killing, exception occurs.

Expected behavior"
No exception?

Environment:
Minimal server setup with Paper 1.20.4 build 461
BentoBox 2.2.0
BSkyblock 1.17.1
DragonFights 1.8.1 (from https://github.com/BONNePlayground/DragonFights/releases)

At the time also had LightEco, Vault, LP, ServerSelectorX and Level addon. (issue occurred without these as well)

Below link is full startup, through to kill of dragon.
https://mclo.gs/xcxI5D2
Exception starts at Line 193 which was the point of killing the dragon (Free the End advancement achieved)