So this may be because UserDefaults are getting reset after a period of time, maybe because I update the OS frequently, but a released version of my app after a period of time will crash on my login screen, probably where I check for saved credentials. Reinstalling the app fixes the issue. Console returns a lot of : Unsupported use of UIKit view-customization API off the main thread. -setAlignsToKeyboard: sent to _UIAlertControllerView: 0x125b2d070; frame = (0 0; 414 896); layer = CALayer: 0x283f652e0 Cannot be called with asCopy = NO on non-main thread. And my system logs show an error on this thread: Exception Type: EXC_CRASH (SIGABRT) Exception Codes: 0x0000000000000000, 0x0000000000000000 Exception Note: EXC_CORPSE_NOTIFY Triggered by Thread: 4 Application Specific Information: abort() called ... Thread 4 name: Dispatch queue: com.apple.NSURLSession-delegate Thread 4 Crashed: 0 libsystem_kernel.dylib 0x00000001bfb4e414 0x1bfb26000 + 164884 1 libsystem_pthread.dylib 0x00000001dd6a8b50 0x1dd6a6000 + 11088 2 libsystem_c.dylib 0x000000019b027b74 0x19afb1000 + 486260 3 libc++abi.dylib 0x00000001a6d1acf8 0x1a6d07000 + 81144 4 libc++abi.dylib 0x00000001a6d0be4c 0x1a6d07000 + 20044 5 libobjc.A.dylib 0x00000001a6c14f64 0x1a6c0e000 + 28516 6 libc++abi.dylib 0x00000001a6d1a0e0 0x1a6d07000 + 78048 7 libc++abi.dylib 0x00000001a6d1a06c 0x1a6d07000 + 77932 8 libdispatch.dylib 0x00000001917eddc4 0x1917ea000 + 15812 9 libdispatch.dylib 0x00000001917f510c 0x1917ea000 + 45324 10 libdispatch.dylib 0x00000001917f5c90 0x1917ea000 + 48272 11 libdispatch.dylib 0x00000001917ffd78 0x1917ea000 + 89464 12 libsystem_pthread.dylib 0x00000001dd6a9814 0x1dd6a6000 + 14356 13 libsystem_pthread.dylib 0x00000001dd6b076c 0x1dd6a6000 + 42860 Here is some code that I use to check saved credentials. In the backend API I cannot see any login attempts. If it helps, I can see this viewcontroller for a split second right before it crashes. Not sure if that rules out viewdidload.    override func viewDidLoad() {     super.viewDidLoad()     addLoadingSpinner()     bLogIn.titleLabel?.font = UIFont.setToVoiceLight()     self.tfCustomerID.delegate = self     self.tfUsername.delegate = self     self.tfPassword.delegate = self     tfUsername.keyboardType = .default     tfPassword.textContentType = .password     setupUI()       if UserDefaults.standard.string(forKey: UserDefaultsKeys.session) != nil       && UserDefaults.standard.string(forKey: UserDefaultsKeys.savedCred) == "True" {         login_session = UserDefaults.standard.string(forKey: UserDefaultsKeys.session)!         check_session()       } else if UserDefaults.standard.string(forKey: UserDefaultsKeys.savedCred) == "True"         && UserDefaults.standard.string(forKey: UserDefaultsKeys.usesBiometrics) == "True" {         promptTouchOrFaceID()       }     removeLoadingSpinner()   }    override func viewWillAppear(_ animated: Bool) {     AppDelegate.AppUtility.lockOrientation(UIInterfaceOrientationMask.portrait, andRotateTo: UIInterfaceOrientation.portrait)     cbSave.isChecked = UserDefaults.standard.string(forKey: UserDefaultsKeys.savedCred) == "True"     addObservers()   }    func check_session() {     UserDefaults.standard.set(login_session, forKey: "session")     UserInfo.access_token = UserDefaults.standard.string(forKey: "session")!           var request = URLRequest(url: NSURL(string: checksession_url)! as URL)     request.httpMethod = .GET     request.addValues(...)     let task = URLSession.shared.dataTask(with: request) { data, response, error in       if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode == 200 { //NEVER GETTING TO API         DispatchQueue.main.async { ...         }       } else {         self.promptTouchOrFaceID()         UserDefaults.standard.set(nil, forKey: UserDefaultsKeys.session)       }     }     task.resume()   } Thanks in advance for any advice. Finding this to be pretty difficult to solve.