My error

[18:57:09] [Server thread/ERROR]: Could not pass event InventoryClickEvent to Quest_World v1.1
org.bukkit.event.EventException
	at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:303) ~[spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:509) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:494) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at net.minecraft.server.v1_8_R1.PlayerConnection.a(PlayerConnection.java:1554) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at net.minecraft.server.v1_8_R1.PacketPlayInWindowClick.a(SourceFile:31) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at net.minecraft.server.v1_8_R1.PacketPlayInWindowClick.a(SourceFile:9) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at net.minecraft.server.v1_8_R1.PacketHandleTask.run(SourceFile:13) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_25]
	at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_25]
	at net.minecraft.server.v1_8_R1.MinecraftServer.z(MinecraftServer.java:686) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at net.minecraft.server.v1_8_R1.DedicatedServer.z(DedicatedServer.java:316) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at net.minecraft.server.v1_8_R1.MinecraftServer.y(MinecraftServer.java:627) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at net.minecraft.server.v1_8_R1.MinecraftServer.run(MinecraftServer.java:530) [spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_25]
Caused by: java.lang.NullPointerException
	at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:68) ~[spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at org.bukkit.inventory.ItemStack.<init>(ItemStack.java:46) ~[spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	at me.mrCookieSlime.QuestWorld.QuestManager.openCategory(QuestManager.java:266) ~[?:?]
	at me.mrCookieSlime.QuestWorld.PlayerStartListener.onClick(PlayerStartListener.java:69) ~[?:?]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_25]
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_25]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_25]
	at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_25]
	at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:299) ~[spigot-1.8-R0.1.jar:git-Spigot-1b9695e]
	... 14 more