title: How to handle iOS 11 Navigation BarButtonItems bug?
body: <p>I found a <code>UINavigationBar</code> bug in iOS 11.Fisrtly setup navigation item button in viewDidLoad with <code>self.navigationItem.rightBarButtonItems = @[fixSpaceItem, item]</code>. And use gesture pop back, but I don't really pop back, when pop back starts, I release my finger and let the viewcontroller cancel pop back, then right navigation button items disappear. The left item buttons have the same problem. To reproduce this bug, you must add fixSpaceItem like <code>[[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil]</code>. And real device but not simulator can reproduce the bug.
  Here is my main code: </p>
  
  <pre><code>- (void)viewDidLoad {
      [super viewDidLoad];
  
      self.navigationController.interactivePopGestureRecognizer.enabled = YES;
      self.navigationController.interactivePopGestureRecognizer.delegate = self;
  
      self.navigationItem.rightBarButtonItems = @[[self negativeSpacerWithWidth:5],[self rightButton]];
      self.navigationItem.leftBarButtonItems = @[[self negativeSpacerWithWidth:5], [self leftButton]];
  
  }
  
  - (UIBarButtonItem *)leftButton {
      UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 35, 35)];
      [button setImage:[UIImage imageNamed:@"icon_app_back_normal"] forState:UIControlStateNormal];
      [button addTarget:self action:@selector(back) forControlEvents:UIControlEventTouchUpInside];
      UIBarButtonItem *item = [[UIBarButtonItem alloc] initWithCustomView:button];
      return item;
  }
  
  - (UIBarButtonItem *)rightButton {
      UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 35, 35)];
      [button setImage:[UIImage imageNamed:@"setting"] forState:UIControlStateNormal];
      UIBarButtonItem *item = [[UIBarButtonItem alloc] initWithCustomView:button];
      return item;
  }
  
  - (UIBarButtonItem *)negativeSpacerWithWidth:(CGFloat)width {
      UIBarButtonItem *spacer = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];
      [spacer setWidth:width];
      return spacer;
  }
  </code></pre>