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Says something about line 76:

package kenneths_mods.kenneths_medieval_mod.init.objects.machines.smeltingfurnace;

import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.entity.player.InventoryPlayer;
import net.minecraft.inventory.Container;
import net.minecraft.inventory.IContainerListener;
import net.minecraft.inventory.Slot;
import net.minecraft.item.ItemStack;
import net.minecraftforge.fml.relauncher.Side;
import net.minecraftforge.fml.relauncher.SideOnly;
import net.minecraftforge.items.CapabilityItemHandler;
import net.minecraftforge.items.IItemHandler;
import net.minecraftforge.items.SlotItemHandler;

public class SmeltingFurnaceContainer extends Container {
	private final SmeltingFurnaceTileEntity tileentity;
	private int cookTime, totalCookTime, burnTime, currentBurnTime;
	public SmeltingFurnaceContainer (InventoryPlayer player, SmeltingFurnaceTileEntity tileentity) {
		this.tileentity = tileentity;
		IItemHandler handler = tileentity.getCapability(CapabilityItemHandler.ITEM_HANDLER_CAPABILITY, null);
		this.addSlotToContainer(new SlotItemHandler(handler, 0, 26, 11));
		this.addSlotToContainer(new SlotItemHandler(handler, 1, 26, 59));
		this.addSlotToContainer(new SlotItemHandler(handler, 2, 7, 35));
		this.addSlotToContainer(new SlotItemHandler(handler, 3, 81, 36));
		
		for(int y = 0; y < 3; y++) {
			for(int x = 0; x < 9; x++) {
				this.addSlotToContainer(new Slot(player, x + y * 9 + 9, 8 + x * 18, 84 + y * 18));
			}
		}
		
		for(int x = 0; x < 9; x++) {
			this.addSlotToContainer(new Slot(player, x, 8 + x * 18, 142));
		}
	}
	
	@Override
	public void detectAndSendChanges() {
		super.detectAndSendChanges();
		for(int i = 0; i < this.listeners.size(); ++i) {
			IContainerListener listener = (IContainerListener)this.listeners.get(i);
			if(this.cookTime != this.tileentity.getField(2)) listener.sendWindowProperty(this, 2, this.tileentity.getField(2));
			if(this.burnTime != this.tileentity.getField(0)) listener.sendWindowProperty(this, 0, this.tileentity.getField(0));
			if(this.currentBurnTime != this.tileentity.getField(1)) listener.sendWindowProperty(this, 1, this.tileentity.getField(1));
			if(this.totalCookTime != this.tileentity.getField(3)) listener.sendWindowProperty(this, 3, this.tileentity.getField(3));
		}
		this.cookTime = this.tileentity.getField(2);
		this.burnTime = this.tileentity.getField(0);
		this.currentBurnTime = this.tileentity.getField(1);
		this.totalCookTime = this.tileentity.getField(3);
	}
	
	@Override
	@SideOnly(Side.CLIENT)
	public void updateProgressBar(int id, int data) {
		this.tileentity.setField(id, data);
	}
	
	@Override
	public boolean canInteractWith(EntityPlayer playerIn) {
		return this.tileentity.isUsableByPlayer(playerIn);
	}
	
	@Override
	public ItemStack transferStackInSlot(EntityPlayer playerIn, int index) {
		ItemStack stack = ItemStack.EMPTY;
		Slot slot = (Slot)this.inventorySlots.get(index);
		
		if(slot != null && slot.getHasStack()) {
			ItemStack stack1 = slot.getStack();
			stack = stack1.copy();
			if(index == 3) {
				if(!this.mergeItemStack(stack1, 4, 40, true)) return ItemStack.EMPTY;
				slot.onSlotChange(stack1, stack);
			} else if (index != 2 && index != 1 && index != 0) {
//Line 76				Slot slot1 = (Slot)this.inventorySlots.get(index + 1);
				if(!SmeltingFurnaceRecipes.getInstance().getSmeltingResult(stack1, slot1.getStack()).isEmpty()) {
					if(!this.mergeItemStack(stack1, 0, 2, false)) {
						return ItemStack.EMPTY;
					} else if (SmeltingFurnaceTileEntity.isItemFuel(stack1)) {
						if(!this.mergeItemStack(stack1, 2, 3, false)) return ItemStack.EMPTY;
					} else if (SmeltingFurnaceTileEntity.isItemFuel(stack1)) {
						if(!this.mergeItemStack(stack1, 2, 3, false)) return ItemStack.EMPTY;
					} else if (SmeltingFurnaceTileEntity.isItemFuel(stack1)) {
						if(!this.mergeItemStack(stack1, 2, 3, false)) return ItemStack.EMPTY;
					} else if (index >= 4 && index < 31) {
						if(!this.mergeItemStack(stack1, 31, 40, false)) return ItemStack.EMPTY;
					} else if (index >= 31 && index < 40 && !this.mergeItemStack(stack1, 4, 31, false)) {
						return ItemStack.EMPTY;
					}
				}
			} else if (!this.mergeItemStack(stack1, 4, 40, false)) {
				return ItemStack.EMPTY;
			}
			if(stack1.isEmpty()) {
				slot.putStack(ItemStack.EMPTY);
			}
			if(stack1.isEmpty()) {
				slot.onSlotChanged();
			}
			if(stack1.getCount() == stack.getCount()) return ItemStack.EMPTY;
			slot.onTake(playerIn, stack1);
		}
		return stack;
	}
}

what is wrong?

For those who need to know I am using the scripts from harrys tech reviews tutorial:
https://github.com/HarryTechRevs/Minecraft-Modding-1.12/blob/master/main/java/harry/mods/tutorial/blocks/containers/ContainerSinteringFurnace.java