Describe the bug
This error is repeatedly appearing on my console
To Reproduce
No specific steps

Expected behavior
No error

[19:02:25 INFO]: Error Unable to intercept a read client packet. (java.lang.RuntimeException: Error occured while instancing generated class.) occured in com.comphenix.protocol.injector.netty.ChannelInjector@98d4d5f.
[19:02:25 ERROR]: [ProtocolLib] INTERNAL ERROR: Unable to intercept a read client packet.
If this problem hasn't already been reported, please open a ticket
at https://github.com/dmulloy2/ProtocolLib/issues with the following data:
Stack Trace:
java.lang.RuntimeException: Error occured while instancing generated class.
at com.comphenix.protocol.reflect.compiler.StructureCompiler.compile(StructureCompiler.java:249)
at com.comphenix.protocol.reflect.compiler.CompiledStructureModifier.withTarget(CompiledStructureModifier.java:132)
at com.comphenix.protocol.events.PacketContainer.(PacketContainer.java:139)
at com.comphenix.protocol.injector.netty.ProtocolInjector.onPacketReceiving(ProtocolInjector.java:324)
at com.comphenix.protocol.injector.netty.ChannelInjector.decode(ChannelInjector.java:593)
at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:489)
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:428)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:265)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at com.comphenix.protocol.injector.netty.ChannelInjector$2.channelRead(ChannelInjector.java:289)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:310)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:284)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at net.minecraft.network.LegacyPingHandler.channelRead(SourceFile:94)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1434)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:965)
at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:808)
at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:404)
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:304)
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:884)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.GeneratedConstructorAccessor244.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.compile(StructureCompiler.java:233)
... 36 more
Caused by: java.lang.NoClassDefFoundError: net/minecraft/server/v1_12_R1/PacketHandshakingInSetProtocol
at com.comphenix.protocol.reflect.compiler.CompiledStructure$net_minecraft_network_handshake_client_C00Handshake$java_lang_Object.(Unknown Source)
... 40 more
Dump:
Parameters:
io.netty.buffer.PooledUnsafeDirectByteBuf@68a01079[
memoryAddress=140385008476176
recyclerHandle=io.netty.util.Recycler$DefaultHandle@73f832e3
chunk=Chunk(1d723f1d: 1%, 57344/16777216)
handle=4611686018427389953
memory=java.nio.DirectByteBuffer[pos=0 lim=16777216 cap=16777216]
offset=8192
length=22
maxLength=32
cache=io.netty.buffer.PoolThreadCache@1435fa26
tmpNioBuf=
allocator=PooledByteBufAllocator(directByDefault: true)
refCnt=1
readerIndex=22
writerIndex=22
markedReaderIndex=0
markedWriterIndex=0
maxCapacity=2147483647
]
manager:
com.comphenix.protocol.injector.PacketFilterManager@5e2d8311[
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@b26057c
packetListeners=[PacketAdapter[plugin=MyCommand, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[UPDATE_SIGN[class=PacketPlayInUpdateSign, id=43]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=WorldGuardExtraFlags, sending=ListeningWhitelist[priority=NORMAL, packets=[REMOVE_ENTITY_EFFECT[class=PacketPlayOutRemoveEntityEffect, id=59]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=ServerListPlus, sending=ListeningWhitelist[priority=NORMAL, packets=[SET_PROTOCOL[class=PacketHandshakingInSetProtocol, id=0], OUT_SERVER_INFO[class=PacketStatusOutServerInfo, id=0]], gamephase=PLAYING, options=[ASYNC]], receiving=ListeningWhitelist[priority=NORMAL, packets=[SET_PROTOCOL[class=PacketHandshakingInSetProtocol, id=0], OUT_SERVER_INFO[class=PacketStatusOutServerInfo, id=0]], gamephase=PLAYING, options=[ASYNC]]], PacketAdapter[plugin=HolographicDisplays, sending=ListeningWhitelist[priority=NORMAL, packets=[ENTITY_METADATA[class=PacketPlayOutEntityMetadata, id=77], SPAWN_ENTITY_LIVING[class=PacketPlayOutSpawnEntityLiving, id=2], SPAWN_ENTITY[class=PacketPlayOutSpawnEntity, id=0], REL_ENTITY_MOVE_LOOK[class=PacketPlayOutRelEntityMoveLook, id=42], REL_ENTITY_MOVE[class=PacketPlayOutRelEntityMove, id=41]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST]]
packetInjector=com.comphenix.protocol.injector.netty.ProtocolInjector$5@31118800
playerInjection=com.comphenix.protocol.injector.netty.ProtocolInjector$4@46e7922e
inputBufferedPackets=[]
recievedListeners=com.comphenix.protocol.injector.SortedPacketListenerList@7460b4e8
sendingListeners=com.comphenix.protocol.injector.SortedPacketListenerList@4f5652fd
hasClosed=false
classLoader=org.bukkit.plugin.java.PluginClassLoader@5587d738
reporter=com.comphenix.protocol.ProtocolLib$1@72ba829f
server=CraftServer{serverName=Mohist,serverVersion=git-Mohist-1.12.2-186,minecraftVersion=1.12.2}
library=ProtocolLib v4.7.0
asyncFilterManager=com.comphenix.protocol.async.AsyncFilterManager@79803a1e
knowsServerPackets=true
knowsClientPackets=true
phaseLoginCount=2
phasePlayingCount=5
packetCreation=false
nettyInjector=com.comphenix.protocol.injector.netty.ProtocolInjector@62d259fa
pluginVerifier=com.comphenix.protocol.injector.PluginVerifier@47b9ccf
hasRecycleDistance=true
minecraftVersion=(MC: 1.12.2)
debug=false
]
Sender:
com.comphenix.protocol.injector.netty.ChannelInjector@98d4d5f[
decodeBuffer=protected void net.minecraft.network.NettyPacketDecoder.decode(io.netty.channel.ChannelHandlerContext,io.netty.buffer.ByteBuf,java.util.List) throws java.lang.Exception
encodeBuffer=protected void net.minecraft.network.NettyPacketEncoder.encode(io.netty.channel.ChannelHandlerContext,java.lang.Object,io.netty.buffer.ByteBuf) throws java.lang.Exception
factory=com.comphenix.protocol.injector.netty.InjectionFactory@4e2c1b73
player=com.comphenix.protocol.injector.server.TemporaryPlayerInvocationHandler@186caf36
updated=
playerName=
playerConnection=
networkManager=net.minecraft.network.NetworkManager@7027479a
originalChannel=[id: 0x17703f0b, L:/172.18.0.2:25689 - R:/50.28.37.175:37778]
channelField=VolatileField [accessor=DefaultFieldAccessor [field=public io.netty.channel.Channel net.minecraft.network.NetworkManager.field_150746_k], container=net.minecraft.network.NetworkManager@7027479a, previous=[id: 0x17703f0b, L:/172.18.0.2:25689 - R:/50.28.37.175:37778], current=com.comphenix.protocol.injector.netty.ChannelInjector$3@217e46e2, previousLoaded=true, currentSet=true, forceAccess=true]
packetMarker={}
currentEvent=
finalEvent=
unfilteredProcessedPackets=com.comphenix.protocol.injector.netty.PacketFilterQueue@5c9648e4
vanillaDecoder=net.minecraft.network.NettyPacketDecoder@1647df7b
vanillaEncoder=net.minecraft.network.NettyPacketEncoder@4cd6140
finishQueue=[]
channelListener=com.comphenix.protocol.injector.netty.ProtocolInjector@62d259fa
processor=com.comphenix.protocol.injector.NetworkProcessor@39e8dd0b
injected=true
closed=false
cumulation=PooledUnsafeDirectByteBuf(ridx: 22, widx: 22, cap: 22)
cumulator=io.netty.handler.codec.ByteToMessageDecoder$1@15fd85e3
singleDecode=false
decodeWasNull=false
first=true
decodeState=1
discardAfterReads=16
numReads=0
added=true
]
Version:
ProtocolLib v4.7.0
Java Version:
1.8.0_282
Server:
git-Mohist-1.12.2-186 (MC: 1.12.2)

The official stance of ProtocolLib is to not use Mohist. They've done some shady stuff with plugin jars which you can read more about here: https://essentialsx.net/do-not-use-mohist.html