Unknown error while moving from one controller to another controller? - ios

Question

In my app i am getting unknown error,while moving from one controller to another by using this code,
-(IBAction)backQA:(id)sender
{
UIAlertController *alert =[UIAlertController alertControllerWithTitle:#"Attention" message:#"Are You Sure Quit The Exam!" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *ok = [UIAlertAction actionWithTitle:#"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
ContestViewController *contestVC =[self.storyboard instantiateViewControllerWithIdentifier:#"ContestVC"];
[self presentViewController:contestVC animated:YES completion:nil];
}];
UIAlertAction *cancel = [UIAlertAction actionWithTitle:#"Cancel" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
}];
[alert addAction:ok];
[alert addAction:cancel];
[self presentViewController:alert animated:YES completion:nil];
}
When i click on my Ok action i am getting error.
Here is the screen shot,
Any help?
Here is the my setMainBanner Method.
- (void)viewDidLoad
{
#try
{
if((NSNull *)[contestDict objectForKey:#"mainbanner"] != [NSNull null])
{
if([[contestDict objectForKey:#"mainbanner"] length] > 0)
{
NSString *mainBannerimage = [contestDict objectForKey:#"mainbanner"];
NSString *bannerImageURL = [NSString stringWithFormat:#"http://www.karo.com/APP/banners/%#", mainBannerimage];
NSURL *imageURL = [NSURL URLWithString:bannerImageURL];
[mainBanner sd_setImageWithURL:imageURL
placeholderImage:[UIImage imageNamed:#"profilepic_bg"]];
}
else
{
}
}
}
#catch (NSException *exception)
{
}
}

Answer 1

Try this:
UIAlertController *alertController = [UIAlertController
alertControllerWithTitle:alertTitle
message:alertMessage
preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *cancelAction = [UIAlertAction
actionWithTitle:NSLocalizedString(#"Cancel", #"Cancel action")
style:UIAlertActionStyleCancel
handler:^(UIAlertAction *action)
{
NSLog(#"Cancel action");
}];
UIAlertAction *okAction = [UIAlertAction
actionWithTitle:NSLocalizedString(#"OK", #"OK action")
style:UIAlertActionStyleDefault
handler:^(UIAlertAction *action)
{
NSLog(#"OK action");
}];
[alertController addAction:cancelAction];
[alertController addAction:okAction];
Finally we can present the alert view controller as with any other view controller:
[self presentViewController:alertController animated:YES completion:nil];

Answer 2

why you are using presentViewController use this
[self dismissViewControllerAnimated:YES completion:nil];