#vulnerable code 
                             private boolean isButtonClicked(InventoryClickEvent event, ItemStack button) {
		if (event.getCurrentItem().isSimilar(button)) {
			// Clicked item seems to be the button. But player could have clicked on item in his personal inventory that
			// matches the properties of the button used by Advanced Achievements. The first item matching the
			// properties of the button is the real one, check that this is indeed the clicked one.
			Map<Integer, ItemStack> backButtonCandidates = new TreeMap<>(
					event.getInventory().all(event.getCurrentItem().getType()));
			for (Entry<Integer, ItemStack> entry : backButtonCandidates.entrySet()) {
				if (event.getCurrentItem().isSimilar(entry.getValue())) {
					// Found real button. Did the player click on it?
					if (entry.getKey() == event.getRawSlot()) {
						return true;
					}
					break;
				}
			}
		}
		return false;
	} 
                            #location 2 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             private boolean isButtonClicked(InventoryClickEvent event, ItemStack button) {
		if (event.getCurrentItem() != null && event.getCurrentItem().isSimilar(button)) {
			// Clicked item seems to be the button. But player could have clicked on item in his personal inventory that
			// matches the properties of the button used by Advanced Achievements. The first item matching the
			// properties of the button is the real one, check that this is indeed the clicked one.
			Map<Integer, ItemStack> backButtonCandidates = new TreeMap<>(
					event.getInventory().all(event.getCurrentItem().getType()));
			for (Entry<Integer, ItemStack> entry : backButtonCandidates.entrySet()) {
				if (event.getCurrentItem().isSimilar(entry.getValue())) {
					// Found real button. Did the player click on it?
					if (entry.getKey() == event.getRawSlot()) {
						return true;
					}
					break;
				}
			}
		}
		return false;
	}