Describe the bug
When I click on my custom item in hand with ItemJoin I got this error.
Stack Trace:
java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @63000207
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:357)
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297)
at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:199)
at java.base/java.lang.reflect.Method.setAccessible(Method.java:193)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.generateClass(StructureCompiler.java:299)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.compile(StructureCompiler.java:213)
at com.comphenix.protocol.reflect.compiler.BackgroundCompiler$2.call(BackgroundCompiler.java:204)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:831)
Dump:
Parameters:
com.comphenix.protocol.reflect.StructureModifier@ecb7f5e[
targetType=class net.minecraft.server.v1_16_R3.PacketPlayInBlockPlace
target=
converter=
fieldType=class java.lang.Object
data=[private net.minecraft.server.v1_16_R3.EnumHand net.minecraft.server.v1_16_R3.PacketPlayInBlockPlace.a, public long net.minecraft.server.v1_16_R3.PacketPlayInBlockPlace.timestamp]
defaultFields={private net.minecraft.server.v1_16_R3.EnumHand net.minecraft.server.v1_16_R3.PacketPlayInBlockPlace.a=0}
subtypeCache={}
customConvertHandling=false
useStructureCompiler=true
]
manager:
com.comphenix.protocol.injector.PacketFilterManager@2fdef123[
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@6bd3d021
packetListeners=[PacketAdapter[plugin=WorldGuardExtraFlags, sending=ListeningWhitelist[priority=NORMAL, packets=[REMOVE_ENTITY_EFFECT[class=PacketPlayOutRemoveEntityEffect, id=55]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=Spartan, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[BLOCK_PLACE[class=PacketPlayInBlockPlace, id=47]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=Spartan, sending=ListeningWhitelist[priority=NORMAL, packets=[KEEP_ALIVE[class=PacketPlayInKeepAlive, id=16], TRANSACTION[class=PacketPlayInTransaction, id=7], KEEP_ALIVE[class=PacketPlayOutKeepAlive, id=31]], gamephase=PLAYING, options=[]], receiving=ListeningWhitelist[priority=NORMAL, packets=[KEEP_ALIVE[class=PacketPlayInKeepAlive, id=16], TRANSACTION[class=PacketPlayInTransaction, id=7], KEEP_ALIVE[class=PacketPlayOutKeepAlive, id=31]], gamephase=PLAYING, options=[]]]]
packetInjector=com.comphenix.protocol.injector.netty.ProtocolInjector$5@2000bc81
playerInjection=com.comphenix.protocol.injector.netty.ProtocolInjector$4@75641650
inputBufferedPackets=[]
recievedListeners=com.comphenix.protocol.injector.SortedPacketListenerList@2d6bb636
sendingListeners=com.comphenix.protocol.injector.SortedPacketListenerList@584c2962
hasClosed=false
classLoader=PluginClassLoader{plugin=ProtocolLib v4.6.0, pluginEnabled=true, url=plugins/ZProtocolLib.jar}
reporter=com.comphenix.protocol.ProtocolLib$1@7d0d625e
server=CraftServer{serverName=Paper,serverVersion=git-Paper-778,minecraftVersion=1.16.5}
library=ProtocolLib v4.6.0
asyncFilterManager=com.comphenix.protocol.async.AsyncFilterManager@5824e408
knowsServerPackets=true
knowsClientPackets=true
phaseLoginCount=0
phasePlayingCount=4
packetCreation=false
nettyInjector=com.comphenix.protocol.injector.netty.ProtocolInjector@515679ef
pluginVerifier=com.comphenix.protocol.injector.PluginVerifier@66710afb
hasRecycleDistance=true
minecraftVersion=(MC: 1.16.5)
debug=false
]
Sender:
com.comphenix.protocol.reflect.compiler.BackgroundCompiler@b8a909d[
listeners={com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@92dc9080=[com.comphenix.protocol.injector.StructureCache$1@4b03b486]}
listenerLock=java.lang.Object@29782508
compiler=com.comphenix.protocol.reflect.compiler.StructureCompiler@47b0c40e
enabled=false
shuttingDown=false
executor=java.util.concurrent.Executors$FinalizableDelegatedExecutorService@18e1ba1d
reporter=com.comphenix.protocol.ProtocolLib$1@7d0d625e
unknownPermGenBean=java.lang.Object@4e109a03
permGenBean=
disablePermGenFraction=0.65
]
Version:
ProtocolLib v4.6.0
Java Version:
16.0.1
Server:
git-Paper-778 (MC: 1.16.5)

To Reproduce
Steps to reproduce the behavior:
I just click on my item in hand, no more.

Screenshots

Version Info
https://pastebin.com/gPi952y1

You may wnne try the latest jenkins Build 521 of Version 4.7
well might this version does not work for Versions older than 1.17..

but i see your using the older release 4.6.0 from the spigot site

Your using Java 16 so you should read the description of the pluginsite it sais:
If you're running Java 16 or Paper dev builds, use 4.6.1.