Local variable class is null

Question

I have one problem. RestKit I use in my application. When I receive data over the network, I keep them in the object and then use the various fields of the class. In my case I get an article in which there is a field of URL, which is used in UIActivity to send a link in the social network. When I call handler function pressing UIAсtivity I get the URL is NULL. I can not understand why this happens.

NewsDetailViewController.h

@property (nonatomic,strong) NSString *URL;

NewsDetailViewController.m

@interface NewsDetailViewController() {
   NSString *URL;
}

@synthesize URL;

[[RKObjectManager sharedManager] getObjectsAtPath:route parameters:params
                                          success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
Article *article = [mappingResult.array firstObject];
 self.URL = article.url;
                                          }
                                          failure:^(RKObjectRequestOperation *operation, NSError *error) {
                                              NSLog(@"Error response': %@", error);
                                          }];

-(void)socialSharing:sender {

dispatch_queue_t queue = dispatch_queue_create("openActivityIndicatorQueue", NULL);

dispatch_async(queue, ^{
    NSArray *itemsToShare = @[self.URL];
    UIActivityViewController *activityVC = [[UIActivityViewController alloc] initWithActivityItems:itemsToShare applicationActivities:nil];
    if (floor(NSFoundationVersionNumber) > NSFoundationVersionNumber_iOS_6_1) {
        activityVC.excludedActivityTypes = @[UIActivityTypeAirDrop];
    }

    dispatch_async(dispatch_get_main_queue(), ^{
        [self presentViewController:activityVC animated:YES completion:nil];
    });
});
}

init button

_items = [[self.toolBar items] mutableCopy];
UIBarButtonItem *shareBtn = [[UIBarButtonItem alloc]     initWithBarButtonSystemItem:UIBarButtonSystemItemAction target:self action:@selector(socialSharing:)];
[_items addObject:shareBtn];
[self.toolBar setItems:_items animated:YES];

Inside function socialSharing:sender my variable self.URL is NULL. I don't understand why?