Bukkit

Plugin Version : 5.3.3
Bukkit/Spigot Build/Version : git-Paper-762

What steps will reproduce the problem?
1. Create two working iconmenus: /help1 and /help2
2. Test them by typing in /help1 and /help2 manually -> works fine!
3. Click an item in /help1 which is intended to open /help2 with - '0:SIGN:0:%openiconmenu%/help2:&bHelp page 2:Open help page 2'

What is the expected output? What do you see instead?
/help2 menu is not opening and error in log:

Do you have an error log of what happened?
[16:47:52] [Server thread/ERROR]: Could not pass event InventoryClickEvent to MyCommand v5.3.2
org.bukkit.event.EventException
    at com.destroystokyo.paper.event.executor.MethodHandleEventExecutor.execute(MethodHandleEventExecutor.java:37) ~[patched_1.9.4.jar:git-Paper-762]
    at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:78) ~[patched_1.9.4.jar:git-Paper-762]
    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[patched_1.9.4.jar:git-Paper-762]
    at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:517) [patched_1.9.4.jar:git-Paper-762]
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:502) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.PlayerConnection.a(PlayerConnection.java:1899) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.PacketPlayInWindowClick.a(SourceFile:33) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.PacketPlayInWindowClick.a(SourceFile:10) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.PlayerConnectionUtils$1.run(SourceFile:13) [patched_1.9.4.jar:git-Paper-762]
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_91]
    at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_91]
    at net.minecraft.server.v1_9_R2.SystemUtils.a(SourceFile:45) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.MinecraftServer.D(MinecraftServer.java:786) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.DedicatedServer.D(DedicatedServer.java:403) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.MinecraftServer.C(MinecraftServer.java:723) [patched_1.9.4.jar:git-Paper-762]
    at net.minecraft.server.v1_9_R2.MinecraftServer.run(MinecraftServer.java:622) [patched_1.9.4.jar:git-Paper-762]
    at java.lang.Thread.run(Unknown Source) [?:1.8.0_91]
Caused by: java.lang.NullPointerException
    at it.mri.mycommand.utilities.IconMenu.onInventoryClick(IconMenu.java:68) ~[?:?]
    at com.destroystokyo.paper.event.executor.MethodHandleEventExecutor.execute(MethodHandleEventExecutor.java:35) ~[patched_1.9.4.jar:git-Paper-762]
    ... 16 more

Please provide any additional information below.
It is confusing that this does not work if an admin (*-permission and op) tests it but it works well if just ops test it (help2 page is opening, no error)