null
org.bukkit.command.CommandException: Unhandled exception executing command 'oitc' in plugin OITC v2.1.2
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot.jar:git-Spigot-1169]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:192) ~[spigot.jar:git-Spigot-1169]
        at org.bukkit.craftbukkit.v1_7_R1.CraftServer.dispatchCommand(CraftServer.java:546) ~[spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.PlayerConnection.handleCommand(PlayerConnection.java:941) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.PlayerConnection.a(PlayerConnection.java:814) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.PacketPlayInChat.a(PacketPlayInChat.java:28) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.PacketPlayInChat.handle(PacketPlayInChat.java:65) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.NetworkManager.a(NetworkManager.java:146) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.ServerConnection.c(SourceFile:134) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.MinecraftServer.u(MinecraftServer.java:647) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.DedicatedServer.u(DedicatedServer.java:259) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.MinecraftServer.t(MinecraftServer.java:530) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.MinecraftServer.run(MinecraftServer.java:442) [spigot.jar:git-Spigot-1169]
        at net.minecraft.server.v1_7_R1.ThreadServerApplication.run(SourceFile:617) [spigot.jar:git-Spigot-1169]
Caused by: java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819) ~[?:1.7.0_25]
        at java.util.ArrayList$Itr.next(ArrayList.java:791) ~[?:1.7.0_25]
        at me.artish1.OITC.Arena.stop(Arena.java:472) ~[?:?]
        at me.artish1.OITC.Arena.removePlayer(Arena.java:621) ~[?:?]
        at me.artish1.OITC.OITC.onCommand(OITC.java:230) ~[?:?]
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot.jar:git-Spigot-1169]
        ... 13 more