I always check the console, and I really get error that I can not find a solution.
I do not know the error.

[22:56:50] [Server thread/ERROR]: Could not pass event InventoryClickEvent to ItemJoin v3.9.1 org.bukkit.event.EventException at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[spigot.jar:git-Spigot-db6de12-18fbb24] at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[spigot.jar:git-Spigot-db6de12-18fbb24] at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [spigot.jar:git-Spigot-db6de12-18fbb24] at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:1630) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.PacketPlayInWindowClick.a(SourceFile:31) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.PacketPlayInWindowClick.a(SourceFile:9) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [spigot.jar:git-Spigot-db6de12-18fbb24] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_111] at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_111] at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [spigot.jar:git-Spigot-db6de12-18fbb24] at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [spigot.jar:git-Spigot-db6de12-18fbb24] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_111] Caused by: java.lang.NullPointerException

Fix it please.
Thank you.

Not quite sure how you are obtaining this error. Do you think you can do indepth more on how this managed to show up on your console?

Also can you please post your config and items.yml into a pastebin.com and link them here. I did some bug testing and wasnt able to come up with anything so its possibly a config issue.