Doing:
else if (args[0].equalsIgnoreCase("test"))

{

final Llama llama = (Llama) ((Player) sender).getWorld().spawnEntity(((Player) sender).getLocation(), EntityType.LLAMA);

llama.getInventory().setDecor(new ItemStack(Material.GREEN_CARPET)); // Problematic code

}

Causes:

Caused by: java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 1
    at java.base/java.util.Arrays$ArrayList.set(Arrays.java:4271) ~[?:?]
    at net.minecraft.core.NonNullList.set(SourceFile:54) ~[spigot-1.20.6-R0.1-SNAPSHOT.jar:4161-Spigot-e2c1eee-6118e53]
    at net.minecraft.world.InventorySubcontainer.a(InventorySubcontainer.java:202) ~[spigot-1.20.6-R0.1-SNAPSHOT.jar:4161-Spigot-e2c1eee-6118e53]
    at org.bukkit.craftbukkit.v1_20_R4.inventory.CraftInventory.setItem(CraftInventory.java:101) ~[spigot-1.20.6-R0.1-SNAPSHOT.jar:4161-Spigot-e2c1eee-6118e53]
    at org.bukkit.craftbukkit.v1_20_R4.inventory.CraftInventoryLlama.setDecor(CraftInventoryLlama.java:20) ~[spigot-1.20.6-R0.1-SNAPSHOT.jar:4161-Spigot-e2c1eee-6118e53]
    at me.hexedhero.pp.commands.PinataPartyCommand.execute(PinataPartyCommand.java:354) ~[?:?]
    at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:149) ~[spigot-api-1.20.6-R0.1-SNAPSHOT.jar:?]
    ... 26 more