Grand Central Dispatch: App crashes on instance member assignment - objective-c

Question

To start with it all works without using the the GCD but I want this happening in a separate thread so trying GCD. I've got a login screen where on pressing the login button i've got the following action:
- (void)login
{
dispatch_queue_t buckyballLoginFetcherQ = dispatch_queue_create("Login Queue", NULL);
dispatch_async(buckyballLoginFetcherQ, ^
{
NSDictionary *resultDictionary = [MyService login:self.name.text password:self.password.text];
self.userDetails = [resultDictionary valueForKey:USER_DETAILS_ATTRIBUTE];
[self performSegueWithIdentifier:#"Login" sender:self];
});
}
In MyService method being called above:
+ (NSDictionary *)executeRequest:(NSDictionary *)requestDictionary
{
// Prepare the URL request and do the following
NSData *results = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:&response error:&urlRequestError];
// Process results
...
}
NOW the bit that crashes:
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
if ([segue.identifier isEqualToString:#"Login"])
{
MyDestinationTableViewController *myDestinationTableViewController = nil;
UITabBarController *tbc = (UITabBarController *)[segue destinationViewController];
for (UIViewController *vc in [tbc viewControllers])
{
if ([vc isKindOfClass:[UINavigationController class]])
{ // in our case all view controlers are navigation controllers :-)
UINavigationController *nc = (UINavigationController *)vc;
if ([[[nc viewControllers] lastObject] isKindOfClass:[BuckyballsTableViewController class]])
{
myDestinationTableViewController = [[nc viewControllers] lastObject];
/**************CRASH LINE************/
buckyballsTableViewController.userDetails = self.userDetails;
}
}
}
}
Again without GCD it works, but it holds up screen so i'd want to do it asynchronously. Is it the instance member causing a problem? OR do i need to use it differently or do more with it? Thank you...

Answer 1

Only the main thread may manipulate the UI, so use this code fragment to call those bits on the main thread:
dispatch_async(dispatch_get_main_queue(), ^{
[self performSegueWithIdentifier:#"Login" sender:self];
});