Bukkit

What steps will reproduce the problem?
1.I did /quake join arena
2.And on join sign it's not working too
3.

What is the expected output? What do you see instead?

What version of the product are you using?
CraftBukkit 1.7.9R0.3
I have MGLibs and MGSkywars installed
I guess this plugins above is not the cause of the proble

Do you have an error log of what happened?
Yes,here it go

[08:51:40] [Server thread/INFO]: gabrieldluca_cn issued server command: /quake join VilaVelha
[08:51:40] [Server thread/WARN]: java.lang.NullPointerException: null value
[08:51:40] [Server thread/WARN]: at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:204)
[08:51:40] [Server thread/WARN]: at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:139)
[08:51:40] [Server thread/WARN]: at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:178)
[08:51:40] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_7_R3.inventory.CraftMetaSkull.serialize(CraftMetaSkull.java:137)
[08:51:40] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_7_R3.inventory.CraftMetaItem.serialize(CraftMetaItem.java:590)
[08:51:40] [Server thread/WARN]: at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSerializable.representData(YamlRepresenter.java:33)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:96)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:164)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:201)
[08:51:40] [Server thread/WARN]: at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSerializable.representData(YamlRepresenter.java:35)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:96)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:164)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:201)
[08:51:40] [Server thread/WARN]: at org.bukkit.configuration.file.YamlRepresenter$RepresentConfigurationSection.representData(YamlRepresenter.java:23)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:96)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.representMapping(BaseRepresenter.java:164)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.SafeRepresenter$RepresentMap.representData(SafeRepresenter.java:201)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.representData(BaseRepresenter.java:96)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.representer.BaseRepresenter.represent(BaseRepresenter.java:66)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.Yaml.dumpAll(Yaml.java:270)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.Yaml.dumpAll(Yaml.java:261)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.Yaml.dumpAll(Yaml.java:233)
[08:51:40] [Server thread/WARN]: at org.yaml.snakeyaml.Yaml.dump(Yaml.java:209)
[08:51:40] [Server thread/WARN]: at org.bukkit.configuration.file.YamlConfiguration.saveToString(YamlConfiguration.java:40)
[08:51:40] [Server thread/WARN]: at org.bukkit.configuration.file.FileConfiguration.save(FileConfiguration.java:103)
[08:51:40] [Server thread/WARN]: at com.Geekpower14.Quake.Arena.APlayer.SaveInventory(APlayer.java:103)
[08:51:40] [Server thread/WARN]: at com.Geekpower14.Quake.Arena.APlayer.<init>(APlayer.java:61)
[08:51:40] [Server thread/WARN]: at com.Geekpower14.Quake.Arena.SArena.joinArena(SArena.java:259)
[08:51:40] [Server thread/WARN]: at com.Geekpower14.Quake.Commands.JoinCommand.onCommand(JoinCommand.java:48)
[08:51:40] [Server thread/WARN]: at com.Geekpower14.Quake.Commands.MyCommandExecutor.onCommand(MyCommandExecutor.java:137)
[08:51:40] [Server thread/WARN]: at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
[08:51:40] [Server thread/WARN]: at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:180)
[08:51:40] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_7_R3.CraftServer.dispatchCommand(CraftServer.java:740)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.PlayerConnection.handleCommand(PlayerConnection.java:956)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.PlayerConnection.a(PlayerConnection.java:817)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.PacketPlayInChat.a(PacketPlayInChat.java:28)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.PacketPlayInChat.handle(PacketPlayInChat.java:47)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.NetworkManager.a(NetworkManager.java:157)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.ServerConnection.c(SourceFile:134)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.MinecraftServer.v(MinecraftServer.java:667)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.DedicatedServer.v(DedicatedServer.java:260)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.MinecraftServer.u(MinecraftServer.java:558)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.MinecraftServer.run(MinecraftServer.java:469)
[08:51:40] [Server thread/WARN]: at net.minecraft.server.v1_7_R3.ThreadServerApplication.run(SourceFile:628)

Please provide any additional information below.

No additional info xD