VERY NEW TO SWIFT fatal error unexpectedly found nil while unwrapping an optional vaule

Question

After more research and reviewing the articles pointed out: I concluded my code was actually correct. I removed the .plist file of my app from the simulator documents folder allowing a new one to be created from my new viewcontrollers.

I am receiving a fatal error. My app does build without error to the simulator; However nothing is displayed and the app crashes. The debugger window states "fata error: unexpectedly found nil while unwrapping an Optional value". I believe the error is because the forKey referring to the lists which references an empty array/

import UIKit

class AllListsViewController: UITableViewController, ListDetailViewControllerDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }



    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {

        return lists.count
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = makeCell(for: tableView)

        let checklist = lists[indexPath.row]
        cell.textLabel!.text = checklist.name
        cell.accessoryType = .detailDisclosureButton

        return cell
    }

    func makeCell(for tableView: UITableView) -> UITableViewCell {
        let cellIdentifier = "Cell"
        if let cell = tableView.dequeueReusableCell(withIdentifier: cellIdentifier) {
            return cell
        } else {
            return UITableViewCell(style: .default, reuseIdentifier: cellIdentifier)
        }
    }

    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        let checklist = lists[indexPath.row]
        performSegue(withIdentifier: "ShowChecklist", sender: checklist)

}

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "ShowChecklist" {
            let controller = segue.destination as! ChecklistViewController
            controller.checklist = sender as! Checklist
        } else if segue.identifier == "AddChecklist" {
            let navigationController = segue.destination
                                                as! UINavigationController
            let controller = navigationController.topViewController
                                                as! ListDetailViewController
            controller.delegate = self
            controller.checklistToEdit = nil
        }
    }



    var lists: [Checklist]


    required init?(coder aDecoder: NSCoder) {

        lists = [Checklist]()
        super.init(coder: aDecoder)
        loadChecklists()
        }


    override func tableView(_ tableView: UITableView,
                            commit editingStyle: UITableViewCellEditingStyle,
                            forRowAt indexPath: IndexPath) {
        lists.remove(at: indexPath.row)
        let indexPaths = [indexPath]
        tableView.deleteRows(at: indexPaths, with: .automatic)
    }

    func listDetailViewControllerDidCancel(
        _ controller: ListDetailViewController) {
        dismiss(animated: true, completion: nil)
    }
    func listDetailViewController(_ controller: ListDetailViewController,
                                  didFinishAdding checklist: Checklist) {
        let newRowIndex = lists.count
        lists.append(checklist)
        let indexPath = IndexPath(row: newRowIndex, section: 0)
        let indexPaths = [indexPath]
        tableView.insertRows(at: indexPaths, with: .automatic)
        dismiss(animated: true, completion: nil)
    }
    func listDetailViewController(_ controller: ListDetailViewController,
                                  didFinishEditing checklist: Checklist) {
        if let index = lists.index(of: checklist) {
            let indexPath = IndexPath(row: index, section: 0)
            if let cell = tableView.cellForRow(at: indexPath) {
                cell.textLabel!.text = checklist.name
            }
        }
        dismiss(animated: true, completion: nil)
    }

    override func tableView(_ tableView: UITableView,
                            accessoryButtonTappedForRowWith indexPath: IndexPath) {

        let navigationController = storyboard!.instantiateViewController(
            withIdentifier: "ListDetailNavigationController")
            as! UINavigationController

        let controller = navigationController.topViewController
                as! ListDetailViewController
        controller.delegate = self

        let checklist = lists[indexPath.row]
        controller.checklistToEdit = checklist

        present(navigationController, animated: true, completion: nil)
    }

    func documentsDirectory() -> URL {
        let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
        return paths[0]
    }

    func dataFilePath() -> URL {
        return documentsDirectory().appendingPathComponent("Checklists.plist")
    }

    func saveChecklists() {
        let data = NSMutableData()
        let archiver = NSKeyedArchiver(forWritingWith: data)

        archiver.encode(lists, forKey: "Checklists")

        archiver.finishEncoding()
        data.write(to: dataFilePath(), atomically: true)
    }

    func loadChecklists() {
        let path = dataFilePath()
        if let data = try? Data(contentsOf: path) {
            let unarchiver = NSKeyedUnarchiver(forReadingWith: data)

            *lists = unarchiver.decodeObject(forKey: "Checklists") as! [Checklist]* ****The Error occurs on this line****

            unarchiver.finishDecoding()
        }
    }
}

Answer 1

as! is explicitly unwrapping the optional. as? will prevent an app crash by simply returning nil if downcasting fails.

if let checklist = (NSKeyedUnarchiver(forReadingWith: data)?.decodeObject(forKey: "Checklists")) as? [Checklist] {
    lists = checklist
}
else {
    // set lists as appropriate
}