App Crashing with EXC_BREAKPOINT Error - ios

Question

I have an app I am making in Xcode 7.2. I have a main menu with a button that has a segue going to a settings view controller. The settings view controller has the following code and every time I get to that view controller it crashes with an EXC_BREAKPOINT error. The funny thing is that this only happens on my iPhone 4 running iOS 7. It does not crash in the iOS Simulator iPhone 6 running iOS 9.
Anyways, here is the code.
//
// SettingsController.swift
// Arbor Hills iOS
//
// Created by Andrew on 11/22/15.
// Copyright © 2015 Arbor Hills Vet. All rights reserved.
//
import UIKit
class SettingsController: UITableViewController {
#IBOutlet weak var username: UITextField!
#IBOutlet weak var password: UITextField!
#IBOutlet weak var notifications: UISwitch!
override func viewDidLoad(){
super.viewDidLoad()
let defaults = NSUserDefaults.standardUserDefaults()
let dusername = defaults.valueForKey("username")!
let dpassword = defaults.valueForKey("password")!
let dnotifications = defaults.valueForKey("notifications")!
username!.text = String(dusername)
password!.text = String(dpassword)
if(dnotifications as! Bool == true){
notifications.setOn(true, animated: true)
}else{
notifications.setOn(false, animated:true)
}
}
#IBAction func saveUsername(sender: AnyObject) {
let defaults = NSUserDefaults.standardUserDefaults()
defaults.setObject(username.text!, forKey: "username")
}
#IBAction func savePassword(sender: AnyObject) {
let defaults = NSUserDefaults.standardUserDefaults()
defaults.setObject(password.text!, forKey: "password")
}
#IBAction func notificationsModified(sender: AnyObject) {
let defaults = NSUserDefaults.standardUserDefaults()
defaults.setObject(notifications.on, forKey: "notifications")
}
}

Answer 1

NSUserDefaults returns nil for an object if no default value is registered. When you unwrap a nil value you'll get an runtime error (crash).
Apple recommends to register a default value for any key as a placeholder until a custom value is saved the first time.
In AppDelegate add in applicationDidFinshLaunching
let defaults = NSUserDefaults.standardUserDefaults()
let defaultValues = ["username" : "", "password" : "", "notifications" : false]
defaults.registerDefaults(defaultValues)
Swift 3+
let defaults = UserDefaults.standard
let defaultValues : [String : Any] = ["username" : "", "password" : "", "notifications" : false]
userDefaults.register(defaults: defaultValues)
That registers empty strings and a boolean false.
Then you can safely retrieve the values
let defaults = NSUserDefaults.standardUserDefaults()
let dusername = defaults.stringForKey("username")!
let dpassword = defaults.stringForKey("password")!
let dnotifications = defaults.boolForKey("notifications")
Swift 3+
let defaults = UserDefaults.standard
let dusername = defaults.string(forKey:"username")!
let dpassword = defaults.string(forKey:"password")!
let dnotifications = defaults.bool(forKey:"notifications")
Use always stringForKey for String values and boolForKey for Bool values.
Never use valueForKey with NSUserDefaults which is a special KVC method.
Then this code could be also simplified:
username!.text = dusername
password!.text = dpassword
notifications.setOn(dnotifications, animated: true)
When writing to user defaults use setBool:forKey for the Bool value, however there is no String equivalent, setObject:forKey is correct.

Answer 2

You should not use valueForKey to fetch objects from NSUserDefaults. Thats a key-value coding method. (See the link on NSKeyValueCoding. you want objectForKey, and you need to check for nil afterwords:
let defaults = NSUserDefaults.standardUserDefaults()
if let dusername = defaults.objectForKey("username") as String
{
//code to use the username
}
else
{
//code to handle the case where there is no username
}