Hidden back button appears sometimes in navigation bar

In my application, I'm using Split view with back button hidden in detail view controller and also I'm doing some background operations. Sometimes when app comes from background to foreground after doing background operations the hidden back button appears on the navigation bar. This happens only when we come to foreground after done with background operations, and it doesn't appeared if we come to foreground and still the background operation is in progress. I don't understand why the hidden back button appears some times even though I hide that. If I tap on that button nothing happens but it still appears. Please help me to solve this issue.

In the RootView of Right Panel, I'm hiding the back button like this

  - (void) viewWillAppear
  {
        self.navigationController.navigationItem.hidesBackButton = YES;
        self.navigationController.navigationItem.leftBarButtonItem = nil;
  }

In didEnterBackground, started background task with a timer

        UIApplication* app = [UIApplication sharedApplication];

        backgroundTask = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:backgroundTask];
        backgroundTask = UIBackgroundTaskInvalid;
    }];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSTimer* t = [NSTimer scheduledTimerWithTimeInterval:INITIAL_BACKGROUND_TIME_LIMIT target:self  selector:@selector(pushPullServer) userInfo:nil repeats:NO];
        [[NSRunLoop currentRunLoop] addTimer:t forMode:NSDefaultRunLoopMode];
        [[NSRunLoop currentRunLoop] run];
    });

After coming from background, In appDidBecomeActive

  - (void) appDidBecomeActive
  {
        self.splitViewController.viewControllers = [NSArray arrayWithObjects:self.tabBar, detailNavigation, nil];
        self.splitViewController.delegate = self;
        self.window.rootViewController = self.splitViewController;
  }