org.bukkit.command.CommandException: Unhandled exception executing command 'level' in plugin PlayerLevels v2.9
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at org.bukkit.craftbukkit.v1_12_R1.CraftServer.dispatchCommand(CraftServer.java:648) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.PlayerConnection.handleCommand(PlayerConnection.java:1397) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.PlayerConnection.a(PlayerConnection.java:1232) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.PacketPlayInChat.a(PacketPlayInChat.java:45) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.PacketPlayInChat.a(PacketPlayInChat.java:1) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_191]
	at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_191]
	at net.minecraft.server.v1_12_R1.SystemUtils.a(SourceFile:46) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.MinecraftServer.D(MinecraftServer.java:748) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.DedicatedServer.D(DedicatedServer.java:406) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.MinecraftServer.C(MinecraftServer.java:679) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:577) [spigot.jar:git-Spigot-eb3d921-b9f1615]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_191]
Caused by: java.lang.NullPointerException: Null string not allowed
	at java.util.Objects.requireNonNull(Unknown Source) ~[?:1.8.0_191]
	at net.minecraft.server.v1_12_R1.NBTTagString.<init>(SourceFile:22) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.NBTTagCompound.setString(SourceFile:141) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at net.minecraft.server.v1_12_R1.GameProfileSerializer.serialize(SourceFile:89) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at org.bukkit.craftbukkit.v1_12_R1.inventory.CraftMetaSkull.applyToItem(CraftMetaSkull.java:84) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at org.bukkit.craftbukkit.v1_12_R1.inventory.CraftItemStack.setItemMeta(CraftItemStack.java:431) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at org.bukkit.craftbukkit.v1_12_R1.inventory.CraftItemStack.asNMSCopy(CraftItemStack.java:45) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at org.bukkit.craftbukkit.v1_12_R1.inventory.CraftInventory.setItem(CraftInventory.java:96) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	at me.arkasher.playerLevel.api.API.openInventoryLevel(API.java:491) ~[?:?]
	at me.arkasher.playerLevel.commands.Level.onCommand(Level.java:32) ~[?:?]
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[spigot.jar:git-Spigot-eb3d921-b9f1615]
	... 15 more