Alertview not called when no Internet access - objective-c

Question

Use the following standard calls to get data from a server - and standard Alert if there's an error - like no internet access. If I turn off the network the app crashes, never hitting the NSLog calls for *response or *error, never entering the alert.
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//this will start the URL call and download in bg
dispatch_async(concurrentQueue, ^{
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithURL:[NSURL URLWithString:#"https://www.websitethatdownloadsdata.com"] completionHandler:^(NSData *myData, NSURLResponse *response, NSError *error) {
NSLog(#"Resp value from NSURL task: %#", response);
NSLog(#"Error value from NSURL task: %#", error);
if (error == nil) {
NSLog(#"Downloading data...");
}
if (error != nil) {
UIAlertController * alert = [UIAlertController alertControllerWithTitle:#"Network Problem" message:#"Cannot download data" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction * actionOK = [UIAlertAction actionWithTitle:#"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
//Here Add Your Action
abort();
}];
[alert addAction:actionOK];
[self presentViewController:alert animated:YES completion:nil];
}

Answer 1

The background queue is redundant because NSURLSession dispatches its tasks on a background thread anyway.
But you have to present the alert controller on the main thread
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithURL:[NSURL URLWithString:#"https://www.websitethatdownloadsdata.com"] completionHandler:^(NSData *myData, NSURLResponse *response, NSError *error) {
NSLog(#"Resp value from NSURL task: %#", response);
NSLog(#"Error value from NSURL task: %#", error);
if (error == nil) {
NSLog(#"Downloading data...");
} else {
UIAlertController * alert = [UIAlertController alertControllerWithTitle:#"Network Problem" message:#"Cannot download data" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction * actionOK = [UIAlertAction actionWithTitle:#"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
//Here Add Your Action
abort();
}];
[alert addAction:actionOK];
dispatch_async(dispatch_get_main_queue()) {
[self presentViewController:alert animated:YES completion:nil];
}
}
}
And you should display also the reason of the error in the NSError instance