How to save data in app

Question

I am new to programming and swift with a little experience in Java and would really appreciate it if you guys could help me with a problem I am having. Currently I am trying to save an array when the app is closed and print it to the log after the app is loaded again but anytime I close it I get the error

fatal error: unexpectedly found nil while unwrapping an Optional value
(lldb) 

Here is the part of the code where I try to save it. It is in the appdelegate file which is where I assume is a correct place to put it.

import UIKit

@UIApplicationMain

class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    let data = NSUserDefaults.standardUserDefaults()

    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        return true
    }
    var savedAssignments = ""
    func applicationWillResignActive(application: UIApplication) {
                      data.setValue(Assignments, forKey: "savedAssignments")
    }

    func applicationDidEnterBackground(application: UIApplication) {
               data.setValue(Assignments, forKey: "savedAssignments")
    }

    func applicationWillEnterForeground(application: UIApplication) {
         }

    func applicationDidBecomeActive(application: UIApplication) {
        print(data)
        print(savedAssignments)
        print(Assignments)
    }
    func applicationWillTerminate(application: UIApplication) {
        }
    }