MythicMobsExtension Console Error

[14:03:37 ERROR]: Encountered an unexpected exception java.util.ConcurrentModificationException: null at java.util.HashMap$HashIterator.nextNode(HashMap.java:1442) ~[?:1.8.0_172] at java.util.HashMap$KeyIterator.next(HashMap.java:1466) ~[?:1.8.0_172] at java.util.AbstractCollection.addAll(AbstractCollection.java:343) ~[?:1.8.0_172] at net.minecraft.server.v1_12_R1.AttributeModifiable.c(SourceFile:62) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.PacketPlayOutUpdateAttributes.<init>(SourceFile:26) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.EntityTrackerEntry.d(EntityTrackerEntry.java:278) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.EntityTrackerEntry.track(EntityTrackerEntry.java:207) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.EntityTracker.updatePlayers(EntityTracker.java:182) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:834) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) ~[craftbukkit.jar:git-Spigot-e8ded36-acbc348] at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [craftbukkit.jar:git-Spigot-e8ded36-acbc348] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_172]