Bukkit

[11:39:22] [Server thread/ERROR]: Could not pass event EntityDeathEvent to Rolecraft v3.1.28
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.event.CraftEventFactory.callEntityDeathEvent(CraftEventFactory.java:392) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityLiving.die(EntityLiving.java:1079) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityZombie.die(EntityZombie.java:429) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.entity.CraftLivingEntity.setHealth(CraftLivingEntity.java:100) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.caliog.Rolecraft.XMechanics.Listeners.DamageListener.onMobDamagedByPlayer(DamageListener.java:201) [Rolecraft.jar:?]
at org.caliog.Rolecraft.XMechanics.Listeners.DamageListener.onDamage(DamageListener.java:100) [Rolecraft.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_111]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_111]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_111]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_111]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:302) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.event.CraftEventFactory.callEvent(CraftEventFactory.java:90) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.event.CraftEventFactory.callEntityDamageEvent(CraftEventFactory.java:575) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.event.CraftEventFactory.handleEntityDamageEvent(CraftEventFactory.java:480) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.event.CraftEventFactory.handleLivingEntityDamageEvent(CraftEventFactory.java:607) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityLiving.damageEntity0(EntityLiving.java:1309) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityLiving.damageEntity(EntityLiving.java:855) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityMonster.damageEntity(EntityMonster.java:44) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityZombie.damageEntity(EntityZombie.java:163) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityHuman.attack(EntityHuman.java:1023) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.EntityPlayer.attack(EntityPlayer.java:1145) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.PlayerConnection.a(PlayerConnection.java:1568) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.PacketPlayInUseEntity.a(SourceFile:69) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.PacketPlayInUseEntity.a(SourceFile:13) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.PlayerConnectionUtils$1.run(SourceFile:13) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_111]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_111]
at net.minecraft.server.v1_11_R1.SystemUtils.a(SourceFile:46) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:739) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:399) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:675) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:574) [spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_111]
Caused by: java.lang.IllegalArgumentException: Cannot drop a Null item.
at org.apache.commons.lang.Validate.notNull(Validate.java:192) ~[spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.CraftWorld.dropItem(CraftWorld.java:284) ~[spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.bukkit.craftbukkit.v1_11_R1.CraftWorld.dropItemNaturally(CraftWorld.java:327) ~[spigot152.jar:git-Spigot-6de2fbc-c1aa859]
at org.caliog.Rolecraft.Entities.Player.Playerface.dropItem(Playerface.java:158) ~[?:?]
at org.caliog.Rolecraft.XMechanics.Listeners.DeathListener.onMobDeath(DeathListener.java:90) ~[?:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_111]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_111]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_111]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_111]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:302) ~[spigot152.jar:git-Spigot-6de2fbc-c1aa859]
... 39 more

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 Test version 1.11 spigot
 Korea language mobs drop item error

Thanks for the report, this will be fixed in the next update.

What du you mean by "Korea language" mobs drop item error ?

Should be fixed in v3.1.29