Alright, so I have created a way of syncing between client and server with packets. It works just fine in game, but when running this code in my event handler:

 

@SubscribeEvent
public void onEntityJoinWorld(EntityJoinWorldEvent event) {
	if(!event.entity.worldObj.isRemote && event.entity instanceof EntityPlayer && ExtendedPlayer.get((EntityPlayer)event.entity) != null) {
		ExtendedPlayer.get((EntityPlayer)event.entity).sync();
	}
}

 

I get this error during me joining:

[01:18:21] [Client thread/FATAL] [FML]: Exception caught executing FutureTask: java.util.concurrent.ExecutionException: java.lang.NullPointerException
java.util.concurrent.ExecutionException: java.lang.NullPointerException
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_65]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_65]
at net.minecraftforge.fml.common.FMLCommonHandler.callFuture(FMLCommonHandler.java:715) [FMLCommonHandler.class:?]
at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:1077) [Minecraft.class:?]
at net.minecraft.client.Minecraft.run(Minecraft.java:376) [Minecraft.class:?]
at net.minecraft.client.main.Main.main(Main.java:117) [Main.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_65]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_65]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_65]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_65]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_65]
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_65]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_65]
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_65]
at net.minecraftforge.gradle.GradleStartCommon.launch(Unknown Source) [start/:?]
at GradleStart.main(Unknown Source) [start/:?]
Caused by: java.lang.NullPointerException
at com.happykiller.weightlimit.player.ExtendedPlayer.get(ExtendedPlayer.java:35) ~[ExtendedPlayer.class:?]
at com.happykiller.weightlimit.server.packet.packets.PacketSyncPlayerProps$Handler$1.run(PacketSyncPlayerProps.java:47) ~[PacketSyncPlayerProps$Handler$1.class:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_65]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_65]
at net.minecraftforge.fml.common.FMLCommonHandler.callFuture(FMLCommonHandler.java:714) ~[FMLCommonHandler.class:?]
... 15 more

 

Here is line 35 of ExtendedPlayer:

public static final ExtendedPlayer get(EntityPlayer player) {
return (ExtendedPlayer)player.getExtendedProperties(EXT_PROP_NAME);
}

 

And here is the packet being sent to update the client:

package com.happykiller.weightlimit.server.packet.packets;

import com.happykiller.weightlimit.player.ExtendedPlayer;

import io.netty.buffer.ByteBuf;
import net.minecraft.client.Minecraft;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.server.MinecraftServer;
import net.minecraft.util.IThreadListener;
import net.minecraftforge.fml.common.network.ByteBufUtils;
import net.minecraftforge.fml.common.network.simpleimpl.IMessage;
import net.minecraftforge.fml.common.network.simpleimpl.IMessageHandler;
import net.minecraftforge.fml.common.network.simpleimpl.MessageContext;

public class PacketSyncPlayerProps implements IMessage {

private NBTTagCompound data;

public PacketSyncPlayerProps() {}

public PacketSyncPlayerProps(EntityPlayer player) {
	data = new NBTTagCompound();

	ExtendedPlayer.get(player).saveNBTData(data);
}

public void fromBytes(ByteBuf buf) {
	data = ByteBufUtils.readTag(buf);
}

public void toBytes(ByteBuf buf) {
	ByteBufUtils.writeTag(buf, data);
}

public static class Handler implements IMessageHandler<PacketSyncPlayerProps, IMessage> {

	public IMessage onMessage(final PacketSyncPlayerProps message, MessageContext ctx) {
		IThreadListener mainThread = Minecraft.getMinecraft();

		final EntityPlayer player = Minecraft.getMinecraft().thePlayer;

		mainThread.addScheduledTask(new Runnable() {
			public void run() {
				System.out.println("Loading client player data...");

				ExtendedPlayer.get(player).loadNBTData(message.data);
			}
		});

		return null;
	}
}
}

 

This might just be me missing something obvious, but I am really flustered on why this is happening. Any thoughts?

Thank you, seems to work now.