[Solved][1.14.4] Unable to add slots that don't belong to the player to the container.

[Cerandior]

I am trying to make a merchant-like entity who will sell stuff to the player. I got the gui displaying correctly and I was about to add the slots to the container.
I don't really need the entity to have an inventory of his own because I am checking only one slot content and based on that I update the other. So it is a very simple interface: https://prnt.sc/rgpua6

Anyway, because I don't want the entity to have an inventory of his own, I created an Inventory field in the entity class passing 2 as the number of slots in the constructor. So when I am adding the slots to the container I pass in this inventory field in the entity. However, I am getting this error:

 

Spoiler

[17:12:11] [Client thread/FATAL] [minecraft/ThreadTaskExecutor]: Error executing task on Client
java.lang.IndexOutOfBoundsException: Index: 36, Size: 36
    at java.util.ArrayList.rangeCheck(ArrayList.java:657) ~[?:1.8.0_181] {}
    at java.util.ArrayList.get(ArrayList.java:433) ~[?:1.8.0_181] {}
    at net.minecraft.inventory.container.Container.getSlot(Container.java:161) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraft.inventory.container.Container.setAll(Container.java:462) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraft.client.network.play.ClientPlayNetHandler.handleWindowItems(ClientPlayNetHandler.java:1164) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at net.minecraft.network.play.server.SWindowItemsPacket.processPacket(SWindowItemsPacket.java:58) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraft.network.play.server.SWindowItemsPacket.processPacket(SWindowItemsPacket.java:13) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraft.network.PacketThreadUtil.lambda$checkThreadAndEnqueue$0(PacketThreadUtil.java:19) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraft.util.concurrent.ThreadTaskExecutor.run(ThreadTaskExecutor.java:140) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.util.concurrent.RecursiveEventLoop.run(RecursiveEventLoop.java:22) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading}
    at net.minecraft.util.concurrent.ThreadTaskExecutor.driveOne(ThreadTaskExecutor.java:110) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.util.concurrent.ThreadTaskExecutor.drainTasks(ThreadTaskExecutor.java:97) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:896) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.Minecraft.run(Minecraft.java:384) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
    at net.minecraft.client.main.Main.main(Main.java:128) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {re:classloading,pl:runtimedistcleaner:A}
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_181] {}
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_181] {}
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_181] {}
    at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_181] {}
    at net.minecraftforge.userdev.FMLUserdevClientLaunchProvider.lambda$launchService$0(FMLUserdevClientLaunchProvider.java:55) ~[forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) [modlauncher-4.1.0.jar:?] {}
    at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) [modlauncher-4.1.0.jar:?] {}
    at net.minecraftforge.userdev.LaunchTesting.main(LaunchTesting.java:101) [forge-1.14.4-28.1.96_mapped_snapshot_20190719-1.14.3-recomp.jar:?] {}

 

Judging by the size of the arraylist that is throwing this error (36), I am guessing it is the inventorySlots arraylist of the container. Its size would be 36 after adding all the player inventory slots. But why is it throwing an IndexOutOfBoundsException ? The addSlot() method doesn't even use #.get(index) anywhere, so where is this happening?

Container Class: https://github.com/arjolpanci/VanillaExtended/blob/master/src/main/java/teabx/vanillaextended/container/WanderingAssassinContainer.java
Entity Class: https://github.com/arjolpanci/VanillaExtended/blob/master/src/main/java/teabx/vanillaextended/entities/WanderingAssassin.java

 

Note: It occurs to me as I am posting this, that the way that I am handling things to get the entity instance in the container (I am using two constructors) is pointless since I have the entity Id passed as the blockpos coordinates when I send a gui request to the client, and I can use that data to get the entity from the world. I will change that later.

Edited March 15, 2020 by Cerandior

[Cerandior]

Fixed by doing what I added in the "Note:" part. 
I still don't know why it wasn't working initially though.