ios - 変数が @(null) になるのを防ぐためにクラッシュを回避するにはどうすればよいですか?

Question

アプリケーションが実行されたときにユーザーの場所を取得しています。場所の変数が @(null) の場合、if ステートメントを防ぎます。My FirstViewController.h:

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    locationManager = [[CLLocationManager alloc] init];
    locationManager.delegate = self;
    locationManager.desiredAccuracy = kCLLocationAccuracyHundredMeters;
    [locationManager startUpdatingLocation];

}

CLLocationManager *locationManager;

- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation
{
    CLLocation *currentLocation = newLocation;
    if (currentLocation != NULL) {
        [(AppDelegate *)[[UIApplication sharedApplication] delegate] setLatitude:[NSString stringWithFormat:@"%.8f", currentLocation.coordinate.latitude]];

        [(AppDelegate *)[[UIApplication sharedApplication] delegate] setLongitude:[NSString stringWithFormat:@"%.8f", currentLocation.coordinate.longitude]];
    }
}

私のSecondViewControllerで：

-(void) GettingGlobalVariables{
    NSString *UsersCurrentLatitude = [(AppDelegate *)[[UIApplication sharedApplication] delegate] latitude];
    NSString *UsersCurrentLongitude = [(AppDelegate *)[[UIApplication sharedApplication] delegate] longitude];

    NSNumberFormatter * singsingsing = [[NSNumberFormatter alloc] init];
    [singsingsing setNumberStyle:NSNumberFormatterDecimalStyle];
    NSNumber * myNumberLatitude = [singsingsing numberFromString:UsersCurrentLatitude];


    NSNumberFormatter * sheCameToMeOneMorning = [[NSNumberFormatter alloc] init];
    [sheCameToMeOneMorning setNumberStyle:NSNumberFormatterDecimalStyle];
    NSNumber * myNumberLongitude = [sheCameToMeOneMorning numberFromString:UsersCurrentLongitude];
        NSLog(@"mynumberlatitude:%@,mynumberlongitude:%@",myNumberLongitude,myNumberLatitude);
    if (!myNumberLatitude) {
        NSLog(@"location variables are null");
        TabBar *TB =[[TabBar alloc]init];
        [TB performSelector:@selector(viewDidLoad)];
        [TB performSelector:@selector(locationManager:didUpdateToLocation:fromLocation:)];
        [self performSelector:@selector(GettingGlobalVariables)];

    }
}

このif文で、変数がnullのときアプリクラッシュ。なにか提案を？