org.bukkit.event.EventException: null
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[spigot.jar:git-Spigot-4bd94dc-3a482bd]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot.jar:git-Spigot-4bd94dc-3a482bd]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:500) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:485) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at org.bukkit.craftbukkit.v1_12_R1.event.CraftEventFactory.callPlayerDeathEvent(CraftEventFactory.java:413) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.EntityPlayer.die(EntityPlayer.java:423) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.EntityLiving.damageEntity(EntityLiving.java:956) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.EntityHuman.damageEntity(EntityHuman.java:759) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.EntityPlayer.damageEntity(EntityPlayer.java:577) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.EntityHuman.attack(EntityHuman.java:1005) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.EntityPlayer.attack(EntityPlayer.java:1302) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.PlayerConnection.a(PlayerConnection.java:1621) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.PacketPlayInUseEntity.a(SourceFile:69) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.PacketPlayInUseEntity.a(SourceFile:13) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_151]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_151]
at net.minecraft.server.v1_12_R1.SystemUtils.a(SourceFile:46) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:748) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [spigot.jar:git-Spigot-4bd94dc-3a482bd]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_151]
Caused by: java.lang.NullPointerException
at one.lindegaard.MobHunting.MobHuntingManager.onMobDeath(MobHuntingManager.java:1790) ~[?:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_151]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_151]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_151]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_151]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:302) ~[spigot.jar:git-Spigot-4bd94dc-3a482bd]
... 22 more

Please show me your PVP section (pvp settings)

pvp:
player:

# Set pvpAllowed=false to disable rewards on killing other players.
pvp_allowed: true

# Set rob_from_victim=true to steal from the victim or 
# rob_from_victim=false to get the reward money from the server.
rob_from_victim: true

# Write the message to the killer, describing the reward / console commands
message: You got {killed_player}'s skull

# The kill prize can be a number to steal x dollars from the killed player,
# or it can be a cut in percent of his balance. Rob from victiom is about where the money comes from.
# If FALSE the money comes from from the server, if TRUE the money comes from the dead player.
# If you dont want the player to get any money for PVP kills, you MUST set pvp_kill_prize: 0
money:
  amount: 1.0%
  chance: 1.0

# One or more console commands to be run when a player kills another player.
commands:
- chance: '0.5'
  cmd: ''

# Drop a head of the killed player
head:
  drophead: true

  # The Head price if you want playerheads to have a value like the bag of gold.
  value: '10'
  chance: 1.0
  message: You got {killed}'s skull

Did that help?

Im still not 100% sure you get this bug. (I dont get the bug myself) but I made a fix, so please test snapshot 600 or newer
http://jenkins.lindegaard.one:8180/job/MobHunting/

fixed in 5.5.7