Program: 2080
Project: 2080
Buggy source:
private void handleNPCItemClicked(int slot){
  npc.getBukkitEntity().getInventory().setItem(slot,previousNPCInv.getItem(slot));
  ItemStack i=npc.getBukkitEntity().getInventory().getItem(slot);
  if (!(npc.getTraderNPC().isBuyable(i.getTypeId()))) {
    player.sendMessage(StringUtils.yellowify(i.getType().name(),ChatColor.RED) + "String_Node_Str");
    return;
  }
  Buyable buyable=npc.getTraderNPC().getBuyable(i.getTypeId());
  int amount=npc.getBukkitEntity().getInventory().getItem(slot).getAmount();
  if (amount - buyable.getBuying().getAmount() <= 0) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  if (!EconomyHandler.canBuy(buyable.getPrice(),player)) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  ItemStack transfer=buyable.getBuying();
  HashMap<Integer,ItemStack> unbought=player.getInventory().addItem(transfer);
  if (unbought.size() >= 1) {
    player.getInventory().setContents(previousPlayerInv.getContents());
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  EconomyHandler.pay(buyable.getPrice(),player);
  player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.GREEN)+ "String_Node_Str"+ StringUtils.yellowify(MessageUtils.getPriceMessage(buyable.getPrice()),ChatColor.GREEN)+ "String_Node_Str");
  npc.getBukkitEntity().getInventory().setContents(sortInventory(npc.getBukkitEntity().getInventory()));
}

Fixed version (ground truth):
private void handleNPCItemClicked(int slot){
  log("String_Node_Str" + checkIfContentsNull(previousNPCInv));
  log("String_Node_Str" + checkIfContentsNull(npc.getBukkitEntity().getInventory()));
  log("String_Node_Str" + (previousNPCInv.getItem(slot) == null || previousNPCInv.getItem(slot).getType() == Material.AIR));
  npc.getBukkitEntity().getInventory().setItem(slot,previousNPCInv.getItem(slot));
  ItemStack i=npc.getBukkitEntity().getInventory().getItem(slot);
  log("String_Node_Str" + (i == null || i.getType() == Material.AIR));
  if (!(npc.getTraderNPC().isBuyable(i.getTypeId()))) {
    player.sendMessage(StringUtils.yellowify(i.getType().name(),ChatColor.RED) + "String_Node_Str");
    return;
  }
  Buyable buyable=npc.getTraderNPC().getBuyable(i.getTypeId());
  int amount=npc.getBukkitEntity().getInventory().getItem(slot).getAmount();
  if (amount - buyable.getBuying().getAmount() <= 0) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  if (!EconomyHandler.canBuy(buyable.getPrice(),player)) {
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  ItemStack transfer=buyable.getBuying();
  HashMap<Integer,ItemStack> unbought=player.getInventory().addItem(transfer);
  if (unbought.size() >= 1) {
    player.getInventory().setContents(previousPlayerInv.getContents());
    player.sendMessage(ChatColor.RED + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.RED)+ "String_Node_Str");
    return;
  }
  EconomyHandler.pay(buyable.getPrice(),player);
  player.sendMessage(ChatColor.GREEN + "String_Node_Str" + StringUtils.yellowify(buyable.getBuying().getAmount() + "String_Node_Str" + buyable.getBuying().getType().name(),ChatColor.GREEN)+ "String_Node_Str"+ StringUtils.yellowify(MessageUtils.getPriceMessage(buyable.getPrice()),ChatColor.GREEN)+ "String_Node_Str");
  npc.getBukkitEntity().getInventory().setContents(sortInventory(npc.getBukkitEntity().getInventory()));
}