Population Density v4.9.1 error

[15:35:48] [Server thread/INFO]: Opped Dukecool15
[15:35:51] [Server thread/INFO]: Dukecool15 issued server command: /visit dukey
[15:35:51] [Server thread/ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'visit' in plugin PopulationDensity v4.9.1
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at org.bukkit.craftbukkit.v1_8_R1.CraftServer.dispatchCommand(CraftServer.java:646) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.PlayerConnection.handleCommand(PlayerConnection.java:1115) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.PlayerConnection.a(PlayerConnection.java:950) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.PacketPlayInChat.a(PacketPlayInChat.java:26) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.PacketPlayInChat.a(PacketPlayInChat.java:53) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.PacketHandleTask.run(SourceFile:13) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_40]
	at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_40]
	at net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:696) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:634) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:537) [spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_40]
Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(Unknown Source) ~[?:1.8.0_40]
	at java.util.HashMap$KeyIterator.next(Unknown Source) ~[?:1.8.0_40]
	at net.minecraft.server.v1_8_R1.EntityTracker.track(EntityTracker.java:32) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.WorldManager.a(WorldManager.java:18) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.World.a(World.java:1065) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.WorldServer.a(WorldServer.java:953) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.World.addEntity(World.java:1058) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.World.addEntity(World.java:986) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at net.minecraft.server.v1_8_R1.PlayerList.moveToWorld(PlayerList.java:569) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at org.bukkit.craftbukkit.v1_8_R1.entity.CraftPlayer.teleport(CraftPlayer.java:475) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at org.bukkit.craftbukkit.v1_8_R1.entity.CraftEntity.teleport(CraftEntity.java:223) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	at me.ryanhamshire.PopulationDensity.PopulationDensity.TeleportPlayer(PopulationDensity.java:862) ~[?:?]
	at me.ryanhamshire.PopulationDensity.PopulationDensity.onCommand(PopulationDensity.java:413) ~[?:?]
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot.jar:git-Spigot-6ee12f6-33d5de3]
	... 14 more