PowerStone Error

[00:28:44] [Server thread/INFO]: PhillyCheezsteak issued server command: /powerstone price 9 /warp Test1
[00:28:44] [Server thread/ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'powerstone' in plugin PowerStones v1.0
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at org.bukkit.craftbukkit.v1_8_R2.CraftServer.dispatchCommand(CraftServer.java:646) ~[spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.PlayerConnection.handleCommand(PlayerConnection.java:1133) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.PlayerConnection.a(PlayerConnection.java:968) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.PacketPlayInChat.a(PacketPlayInChat.java:45) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.PacketPlayInChat.a(PacketPlayInChat.java:1) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.PlayerConnectionUtils$1.run(SourceFile:13) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_31]
	at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_31]
	at net.minecraft.server.v1_8_R2.SystemUtils.a(SourceFile:60) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.MinecraftServer.A(MinecraftServer.java:710) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.DedicatedServer.A(DedicatedServer.java:368) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.MinecraftServer.z(MinecraftServer.java:651) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at net.minecraft.server.v1_8_R2.MinecraftServer.run(MinecraftServer.java:554) [spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_31]
Caused by: java.lang.UnsupportedOperationException
	at java.util.AbstractList.remove(Unknown Source) ~[?:1.8.0_31]
	at plugin.PowerStones.price(PowerStones.java:94) ~[?:?]
	at plugin.PowerStones.onCommand(PowerStones.java:65) ~[?:?]
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot-latest.jar:git-Spigot-ea179b3-6e0120a]
	... 15 more