Please Help me! Bug in my Code

I coded a plugin which is supposed to be used for trading with other players.
Whatever here's my code:

Code:

@EventHandler
    public void InventoryClickEvent (InventoryClickEvent ev) {
        System.out.println("step 1");
        Location loc = ev.getInventory().getLocation();
        World w = ((Dropper) ev.getClickedInventory()).getWorld();
        Block bl = w.getBlockAt(loc);
        String fckdch = ev.getClickedInventory().getTitle();
        if(fckdch == "container.dropper" ) {
            System.out.println("step2");
            System.out.println(ev.getAction());
            if(ev.getAction() == InventoryAction.PLACE_ALL) {
                System.out.println("step3");
                int blx = bl.getX();
                int bly = bl.getY();
                int blz = bl.getZ();
                Block bls = w.getBlockAt(blx, bly, blz - 1);
                Block blu = w.getBlockAt(blx, bly - 1, blz);
                if (blu instanceof Chest) {
                    System.out.println("step4");
                    if (bls instanceof Sign) {
                        System.out.println("step5");
                        String cost = ((Sign) bls).getLine(1);
                        String id = ((Sign) bls).getLine(2);
                        String amount = ((Sign) bls).getLine(3);
                        int costint = Integer.parseInt(cost);
                        int amountint = Integer.parseInt(amount);
                        if(ev.getCurrentItem().getType() == Material.DIAMOND){
                            System.out.println("step6");
                            int payment = ev.getCurrentItem().getAmount();
                            int transactions = payment/costint;
                            int paymentdel = transactions * costint;
                            int paymentback = payment - paymentdel;
                            int transactionamount = amountint * transactions;
                            Inventory bluinv = ((Chest) blu).getInventory();
                            boolean accessible = bluinv.contains(Material.DIAMOND, transactionamount);
                            if(accessible == true){
                                System.out.println("step7");
                                Inventory blinv = ((Dropper) bl).getInventory();
                                blinv.clear();
                                blinv.addItem(new ItemStack(Material.DIAMOND, paymentback));
                                blinv.addItem(new ItemStack(Material.valueOf(id), transactionamount));
                                bluinv.removeItem(new ItemStack(Material.valueOf(id),transactionamount));
                            }

                        }
                    }
                }
            }
        }
    }

I know I'm not a genius with this code and there are thousands better ways to do what I want to do here, but I don't care about that I'm still learning.
When i tried it on my Server I got this message after triggering the event:

I can't look what are the 15 more's but I hope you guys can help me anyways!

Thanks a lot for looking at this and hopefully for answers!

Ohh and I'm pretty sure the fault is in the first few lines

Code:

System.out.println("step 1");
        Location loc = ev.getInventory().getLocation();
        World w = ((Dropper) ev.getClickedInventory()).getWorld();
        Block bl = w.getBlockAt(loc);
        String fckdch = ev.getClickedInventory().getTitle();

 

@xXblack_lionXx

You need to make sure the inventory instance is an instance of Dropper before casting it to a Dropper. Currently, you are casting it regardless of if it is a dropper.

 

Thanks a lot!!
Such a stupid mistake xD

 

Please mark this thread as solved if this fixed you problem.

 

how do I do that?