Error message

[06:36:18 ERROR]: Could not pass event CreatureSpawnEvent to InsaneMobs2 v2.0
org.bukkit.event.EventException: null
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:499) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:484) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at org.bukkit.craftbukkit.v1_12_R1.event.CraftEventFactory.callCreatureSpawnEvent(CraftEventFactory.java:283) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.World.addEntity(World.java:1042) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.WorldServer.addEntity(WorldServer.java:1112) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.SpawnerCreature.a(SpawnerCreature.java:188) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.WorldServer.doTick(WorldServer.java:286) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:801) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at java.lang.Thread.run(Thread.java:804) [?:?]
Caused by: java.lang.UnsupportedOperationException: Not supported.
        at org.bukkit.craftbukkit.v1_12_R1.entity.CraftSkeleton.setSkeletonType(CraftSkeleton.java:36) ~[spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        at me.emeralddev.insanemobs2.mobs.darkknight.spawn(darkknight.java:81) ~[?:?]
        at me.emeralddev.insanemobs2.naturalspawn.onCreatureSpawn(naturalspawn.java:167) ~[?:?]
        at sun.reflect.GeneratedMethodAccessor68.invoke(Unknown Source) ~[?:?]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
        at java.lang.reflect.Method.invoke(Method.java:531) ~[?:?]
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:302) ~[spigot-1.12.1.jar:git-Spigot-edd0396-20ab5d3]
        ... 13 more