The STRANGEST error in Swift Collection View - ios

Question

Im facing the strangest problem i have experienced using Swift, Im developing a game and im using a collection view cell to show the different add ons that ill have, like clothes and stuff, however, im loading the image names that every collectionviewcell is going to have within an String List, The problem appears when I want to get a check image inside the selected "thing" when i click in one cell, and if i click in other cell, the check goes to that by first deselecting all the cells, everything works fine! but when i put more than 8 items in the String list the following error appears "fatal error: unexpectedly found nil while unwrapping an Optional value" its weird because the items are shown in the collection view fine but the problem appears when i click in one of them, with more than 8 items I repeat.
This is my view controller:
//
// WeaponSelectorViewController.swift
// swiftSpriteAwesomeProject
//
// Created by Mauri C. on 3/2/15.
// Copyright (c) 2015 WANT. All rights reserved.
//
import UIKit
class WeaponSelectorViewController: UIViewController {
var images = [NSString]()
let defaults = NSUserDefaults.standardUserDefaults()
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
images = ["chancla", "chancle", "chancle2", "chancle3", "chancle4", "chancle5", "chancle7"]
images.append("chancla")
images.append("chancla")
}
func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return images.count
}
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
var cell = collectionView.dequeueReusableCellWithReuseIdentifier("CollectionViewCell", forIndexPath: indexPath) as WeaponCollectionViewCell
cell.checkImageView.hidden = true
if(defaults.stringForKey("chancla") != nil){
if(defaults.stringForKey("chancla")! == images[indexPath.row]){
cell.checkImageView.hidden = false
cell.checkImageView.image = UIImage(named: "check")
}
}
cell.imageView.image = UIImage(named:images[indexPath.row])
cell.imageView.contentMode = UIViewContentMode.ScaleAspectFit
return cell
}
// Uncomment this method to specify if the specified item should be selected
func collectionView(collectionView: UICollectionView, shouldSelectItemAtIndexPath indexPath: NSIndexPath) -> Bool {
NSLog("\(indexPath)")
let cell = collectionView.cellForItemAtIndexPath(indexPath) as WeaponCollectionViewCell
NSLog("\(self.images.count)")
for var i = 0; i < self.images.count; i++ {
var celli = collectionView.cellForItemAtIndexPath(NSIndexPath(forItem: i, inSection: 0)) as WeaponCollectionViewCell
celli.checkImageView.hidden = true
}
if(cell.checkImageView.hidden){
cell.checkImageView.hidden = false
cell.checkImageView.image = UIImage(named: "check")
defaults.setObject(images[indexPath.row], forKey: "chancla")
} else {
cell.checkImageView.hidden = true
}
return true
}
}