Works on one machine but unexpectedly found nil while unwrapping an Optional value on another

Question

My friend and I are working on an app using git. We've encountered a strange problem, when the code runs on my computer but doesn't on hers. Here's the code:

import UIKit

class SelectionViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {


    @IBOutlet weak var tableView: UITableView!

    override func viewDidLoad() {
        super.viewDidLoad()

        //adding table
        self.tableView.registerClass(UITableViewCell.self, forCellReuseIdentifier: "cell")

        // HTTP POST
        let myURL = NSURL(string: "http://.../drinks.php")

        let request = NSMutableURLRequest(URL:myURL!)

        request.HTTPMethod = "POST"

        let postString = ""

        print(postString)

        request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)

        NSURLSession.sharedSession().dataTaskWithRequest(request, completionHandler: {
            (data: NSData?, response: NSURLResponse?, error: NSError?) -> Void in


            dispatch_async(dispatch_get_main_queue())
            {

                do {
                    let json = try NSJSONSerialization.JSONObjectWithData(data!, options: .MutableContainers) as? NSDictionary

                    if let parseJSON = json {


                        // Store the variables locally
                        NSUserDefaults.standardUserDefaults().setObject(parseJSON["Selection"], forKey: "items")
                        NSUserDefaults.standardUserDefaults().setObject(parseJSON["Details"], forKey: "details")
                        //NSUserDefaults.standardUserDefaults().synchronize()
                    }
                } catch
                {
                    print(error)
                }
            }
        }).resume()
    }

    // Grab the values from the local storage and declare them locally
    var details = (NSUserDefaults.standardUserDefaults().stringArrayForKey("details"))!
    var items = (NSUserDefaults.standardUserDefaults().stringArrayForKey("items"))!

It breaks on the last two lines saying: fatal error: unexpectedly found nil while unwrapping an Optional value:

    var details = (NSUserDefaults.standardUserDefaults().stringArrayForKey("details"))!
    var items = (NSUserDefaults.standardUserDefaults().stringArrayForKey("items"))!

Any help will be highly appreciated!