Swift: Unknown error - Unexpectedly found nil while unwrapping an Optional value

Question

I know this is a common error people post here but I can't find a post that matches to what I'm doing even if its the same fundamentally. I'm new to Swift and just trying to find my way, thank you.

The first time I open my app, a blog reader app that reads from a MYSQL database, it works as intended, I can follow the blogs that I chose and unfollow. When I follow a blog/cell it saves to User Defaults using KeyArchiver but when I double tap the home button to clear the app from memory and reopen the app, it crashes.

Something wrong is going on in my loadUserDefaults because I set up breakpoints and it crashes at this line self.followedIdentifiers = Set(UserDefaults.standard.stringArray(forKey: "followedID")!)

I know I have an optional but why is it crashing/ coming back nil if I saved it with saveUserDefaults. Is it not saving? or am I not loading it correctly?

The error is this

fatal error: unexpectedly found nil while unwrapping an Optional value

Code: This is MainController.swift

var mainArray = [Blog]()
var followedArray = [Blog]()
var filteredArray = [Blog]()
var followedIdentifiers = Set<String>()

override func viewDidLoad() {
    super.viewDidLoad()

    // Receiving Data from Server
    retrieveDataFromServer()

    // NSCoding - Unarchiving Data (followedID)
    loadUserDefaults()
}

// NSCoding: Archiving UserDefaults
func saveUserDefaults() {

    // Saving to UserDefaults
    let encodedData = NSKeyedArchiver.archivedData(withRootObject: self.followedIdentifiers)
    UserDefaults.standard.setValue(encodedData, forKey: "followedID")
    UserDefaults.standard.synchronize()
}

// NSCoding: Unarchiving UserDefaults
func loadUserDefaults() { // --- Crash is Here ---

    // Unarchiving Data
    if let data = UserDefaults.standard.data(forKey: "followedID"), let myFollowedList = NSKeyedUnarchiver.unarchiveObject(with: data) as? Set<String> {

        self.followedIdentifiers = myFollowedList
        self.followedIdentifiers = Set(UserDefaults.standard.stringArray(forKey: "followedID")!) // CRASH

    } else {
        print("Error/ Empty: (Loading UserDefaults (followedID))")
    }
}

// Retrieving Data from Server
func retrieveDataFromServer() {

    let getDataURL = "http://example.com/receiving.php"
    let url: NSURL = NSURL(string: getDataURL)!

    do {
        let data: Data = try Data(contentsOf: url as URL)
        let jsonArray = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! NSMutableArray

        // Clear the arrays
        self.followedArray = [Blog]()
        self.mainArray = [Blog]()

        // Looping through jsonArray
        for jsonObject in jsonArray {

            if let blog = Blog(jsonObject:jsonObject as! [String : Any]) {

                // Check if identifiers match
                if followedIdentifiers.contains(blog.blogID) {
                    self.followedArray.append(blog)
                } else {
                    self.mainArray.append(blog)
                }
            }
        }
    } catch {
        print("Error: (Retrieving Data)")
    }
    myTableView.reloadData()
}

This is Blog.swift which handles all the blogs objects and NSCoding

class Blog: NSObject, NSCoding {

var blogName: String
var blogStatus1: String
var blogStatus2: String
var blogURL: String
var blogID: String
var blogType: String
var blogDate: String
var blogPop: String


private init (name: String,status1: String,status2: String,url: String,id: String,type: String,date: String,pop: String) {
    blogName = name
    blogStatus1 = status1
    blogStatus2 = status2
    blogURL = url
    blogID = id
    blogType = type
    blogDate = date
    blogPop = pop
    super.init()
}

convenience init?(jsonObject: [String:Any]) {

    guard let bID = jsonObject["id"] as? String,
        let bName = jsonObject["blogName"] as? String,
        let bStatus1 = jsonObject["blogStatus1"] as? String,
        let bStatus2 = jsonObject["blogStatus2"] as? String,
        let bURL = jsonObject["blogURL"] as? String,
        let bType = jsonObject["blogType"] as? String,
        let bDate = jsonObject["blogDate"] as? String,
        let bPop = jsonObject["blogPop"] as? String

        else {
            print("Error: (Creating Blog Object)")
            return nil
    }

    self.init(name: bName, status1: bStatus1, status2: bStatus2, url: bURL, id: bID, type: bType, date: bDate, pop: bPop)

}

convenience required init?(coder aDecoder: NSCoder) {
    guard let blogName = aDecoder.decodeObject(forKey: "blogName") as? String,
        let blogStatus1 = aDecoder.decodeObject(forKey: "blogStatus1") as? String,
        let blogStatus2 = aDecoder.decodeObject(forKey: "blogStatus2") as? String,
        let blogURL = aDecoder.decodeObject(forKey: "blogURL") as? String,
        let blogID = aDecoder.decodeObject(forKey: "blogID") as? String,
        let blogType = aDecoder.decodeObject(forKey: "blogType") as? String,
        let blogDate = aDecoder.decodeObject(forKey: "blogDate") as? String,
        let blogPop = aDecoder.decodeObject(forKey: "blogPop") as? String else {
            print("Error: (Creating Blog Object)")
            return nil
    }
    self.init(name: blogName, status1: blogStatus1, status2: blogStatus2, url: blogURL, id: blogID, type: blogType, date: blogDate, pop: blogPop)
}

func encode(with aCoder: NSCoder) {
    aCoder.encode(blogName, forKey: "blogName")
    aCoder.encode(blogStatus1, forKey: "blogStatus1")
    aCoder.encode(blogStatus2, forKey: "blogStatus2")
    aCoder.encode(blogURL, forKey: "blogURL")
    aCoder.encode(blogID, forKey: "blogID")
    aCoder.encode(blogType, forKey: "blogType")
    aCoder.encode(blogDate, forKey: "blogDate")
    aCoder.encode(blogPop, forKey: "blogPop")
 }
}

Answer 1

I used File Manager to save my archived array and it was the best choice, so easy and simple. Using Swift 3. Credit to @GuyKogus, @Paulw11 & @MartinR

Used to save my object

NSKeyedArchiver.archiveRootObject(myObject, toFile: filePath)

Used to load my object

if let myFollowedList = NSKeyedUnarchiver.unarchiveObject(withFile: filePath) as? Set<String> {

    myObject = myFollowedList
}