Console Error when hitting mounted mobs

So, I created a mounted mob. It functions fine as intended.

However, the console sends an error whenever I hit the mob.
No errors are present once the rider dies and the remaining mob continues combat.

I am using the latest MythicMobs Dev Build and the latest Spigot version.

Error: https://pastebin.com/MWM9dVuA
Config: https://pastebin.com/arcf94fk