App terminates on back button - iphone

Question

I have a view controller "Gallery screen" namely, as user taps button "Gallery" it goes to that view controller. Now I have a web service call if any picture of particular outlet exists fetch it or else if it doesn't exist then show an AlertView saying "No image found". Now my problem is when user go to gallery screen if there is no image of that outlet and user hits back button before response of service "no image found" app terminates. I get that popup saying "no image found" on previous screen that i get after pressing back button as i "Ok" it, it terminates but THIS ONLY HAPPENS IN iOS 7. Please help and thanks in advance.
EDIT:
I have also tried to dealloc my objects but still the same issue. May be I'm missing something here is the code that I used.
-(void)dealloc
{
bllGalleryImages.delegate = nil;
bllGalleryImages = nil;
}
here is my viewDidLoad Method
-(void)viewDidLoad
{
[super viewDidLoad];
progress = [[MBProgressHUD alloc] init];
progress.labelText = #"Loading";
[progress showUsingAnimation:YES];
[self.view addSubview:progress];
scrl = [[UIScrollView alloc] initWithFrame:CGRectMake(10, 100, 300, 300)];
scrl.scrollEnabled = YES;
scrl.minimumZoomScale=1.0;
scrl.maximumZoomScale=4.0;
scrl.delegate=self;
// Do any additional setup after loading the view from its nib.
}
here is my View will appear method
-(void)viewWillAppear:(BOOL)animated
{
[self userInterface];
[self callService];
}
here is my callService method
-(void)callService
{
NSMutableDictionary *param = [[NSMutableDictionary alloc] init];
[param setObject:#"xyz" forKey:#"DeveloperKey"];
[param setObject:outletObjForInfo.outlet_id forKey:#"outletID"];
bllGalleryImages = [[BLLAuthentication alloc] init];
bllGalleryImages.delegate = self;
[bllGalleryImages getGalleryImages:param];
}