I looked at someone else's code, and was able to kind of figure it out, but I'm getting IndexOutOfBound Exceptions. 
 

public static class Factory implements IContainerFactory<CustomContainer> {

			@Override
			public CustomContainer create(int windowId, PlayerInventory playerInv, PacketBuffer extraData) {
				return new CustomContainer(windowId, getEntityFromBuf(extraData));
			}

			private CustomEntity getEntityFromBuf(PacketBuffer buf) {
				if (buf == null) {
					return null;
				}
				return DistExecutor.unsafeCallWhenOn(Dist.CLIENT, () -> () -> {
					Entity entity = Minecraft.getInstance().world.getEntityByID(buf.readVarInt());
					if (entity instanceof CustomEntity) {
						return (CustomEntity) entity;
					}
					return null;
				});
			}
		}

In case this is needed: 

public CustomContainer(int id, CustomEntity en) {
			super(RegistryHandler.Custom_CONTAINER.get(), id);
			this.en = en;
		}

And this is the stack trace

[m[1;31m[20:13:06] [Render thread/FATAL] [minecraft/ThreadTaskExecutor]: Error executing task on Client
java.lang.RuntimeException: java.lang.IndexOutOfBoundsException
	at net.minecraftforge.fml.DistExecutor.unsafeCallWhenOn(DistExecutor.java:76) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at com.david.test.entities.CustomEntity$CustomContainer$Factory.getEntityFromBuf(CustomEntity.java:212) ~[main/:?] {re:classloading}
	at com.david.test.entities.CustomEntity$CustomContainer$Factory.create(CustomEntity.java:205) ~[main/:?] {re:classloading}
	at com.david.test.entities.CustomEntity$CustomContainer$Factory.create(CustomEntity.java:1) ~[main/:?] {re:classloading}
	at net.minecraft.inventory.container.ContainerType.create(ContainerType.java:53) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:accesstransformer:B}
	at net.minecraftforge.fml.network.FMLPlayMessages$OpenContainer.lambda$handle$0(FMLPlayMessages.java:285) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at java.util.Optional.ifPresent(Optional.java:183) ~[?:?] {}
	at net.minecraftforge.fml.network.FMLPlayMessages$OpenContainer.lambda$handle$1(FMLPlayMessages.java:284) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraftforge.fml.network.NetworkEvent$Context.enqueueWork(NetworkEvent.java:215) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraftforge.fml.network.FMLPlayMessages$OpenContainer.handle(FMLPlayMessages.java:282) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraftforge.fml.network.simple.IndexedMessageCodec.lambda$tryDecode$3(IndexedMessageCodec.java:128) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at java.util.Optional.ifPresent(Optional.java:183) ~[?:?] {}
	at net.minecraftforge.fml.network.simple.IndexedMessageCodec.tryDecode(IndexedMessageCodec.java:128) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraftforge.fml.network.simple.IndexedMessageCodec.consume(IndexedMessageCodec.java:162) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraftforge.fml.network.simple.SimpleChannel.networkEventListener(SimpleChannel.java:80) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraftforge.eventbus.EventBus.doCastFilter(EventBus.java:212) ~[eventbus-2.2.0-service.jar:?] {}
	at net.minecraftforge.eventbus.EventBus.lambda$addListener$11(EventBus.java:204) ~[eventbus-2.2.0-service.jar:?] {}
	at net.minecraftforge.eventbus.EventBus.post(EventBus.java:258) ~[eventbus-2.2.0-service.jar:?] {}
	at net.minecraftforge.fml.network.NetworkInstance.dispatch(NetworkInstance.java:84) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraftforge.fml.network.NetworkHooks.lambda$onCustomPayload$1(NetworkHooks.java:73) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at java.util.Optional.map(Optional.java:265) ~[?:?] {}
	at net.minecraftforge.fml.network.NetworkHooks.onCustomPayload(NetworkHooks.java:73) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraft.client.network.play.ClientPlayNetHandler.handleCustomPayload(ClientPlayNetHandler.java:2076) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
	at net.minecraft.network.play.server.SCustomPayloadPlayPacket.processPacket(SCustomPayloadPlayPacket.java:68) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraft.network.play.server.SCustomPayloadPlayPacket.processPacket(SCustomPayloadPlayPacket.java:11) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraft.network.PacketThreadUtil.lambda$checkThreadAndEnqueue$0(PacketThreadUtil.java:19) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraft.util.concurrent.ThreadTaskExecutor.run(ThreadTaskExecutor.java:139) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:accesstransformer:B}
	at net.minecraft.util.concurrent.RecursiveEventLoop.run(RecursiveEventLoop.java:22) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraft.util.concurrent.ThreadTaskExecutor.driveOne(ThreadTaskExecutor.java:109) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:accesstransformer:B}
	at net.minecraft.util.concurrent.ThreadTaskExecutor.drainTasks(ThreadTaskExecutor.java:97) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:accesstransformer:B}
	at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:959) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.client.Minecraft.run(Minecraft.java:587) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.client.main.Main.main(Main.java:184) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?] {}
	at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?] {}
	at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?] {}
	at java.lang.reflect.Method.invoke(Method.java:567) ~[?:?] {}
	at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {}
	at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-5.1.2.jar:?] {}
	at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-5.1.2.jar:?] {}
	at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-5.1.2.jar:?] {}
	at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) [modlauncher-5.1.2.jar:?] {}
	at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) [modlauncher-5.1.2.jar:?] {}
	at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:105) [forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {}
Caused by: java.lang.IndexOutOfBoundsException
	at io.netty.buffer.EmptyByteBuf.readByte(EmptyByteBuf.java:531) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
	at net.minecraft.network.PacketBuffer.readByte(PacketBuffer.java:790) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at net.minecraft.network.PacketBuffer.readVarInt(PacketBuffer.java:213) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	at com.david.test.entities.CustomEntity$CustomContainer$Factory.lambda$1(CustomEntity.java:213) ~[main/:?] {re:classloading}
	at net.minecraftforge.fml.DistExecutor.unsafeCallWhenOn(DistExecutor.java:72) ~[forge-1.16.1-32.0.75_mapped_snapshot_20200514-1.16-recomp.jar:?] {re:classloading}
	... 43 more

I'm assuming its a problem with the method getting the CustomEntity from a PacketBuffer, but I have no idea what the problem is. 

Edited August 1, 20205 yr by DavidQF555

• Give the code for when you open the Gui (the call to `NetworkHooks#openGui`). Your error is most likely from either not writing the data to the packet buffer, or calling the read and write methods of mismatched types (writeInt and readVarInt are two different methods with different effects).
   
• You do not need to use DistExecutor. You already have an instance of PlayerInventory, which has a PlayerEntity in the `player` variable, which has the player's World in the `world` variable.
   
• I suggest you inline your `getEntityFromBuf` method into the `create` method if there are no other classes calling that method, as it is unnecessary. I also suggest double-checking your code in CustomContainer, if there are places that will error if `null` is the entity passed into the constructor.

I was able to solve it, for some reason I just solve these problems like an hour after posting

How did you solve it @DavidQF555?