Fix react view button flicking (#5301)

lib/ios/RNNUIBarButtonItem.m

 -(instancetype)init:(NSString*)buttonId withCustomView:(RCTRootView *)reactView {
 	self = [super initWithCustomView:reactView];
 	
+	reactView.hidden = YES;
 	reactView.sizeFlexibility = RCTRootViewSizeFlexibilityWidthAndHeight;
 	reactView.delegate = self;
 	reactView.backgroundColor = [UIColor clearColor];
 }
 
 - (void)rootViewDidChangeIntrinsicSize:(RCTRootView *)rootView {
+	rootView.hidden = NO;
 	self.widthConstraint.constant = rootView.intrinsicContentSize.width;
 	self.heightConstraint.constant = rootView.intrinsicContentSize.height;
 	[rootView setFrame:CGRectMake(0, 0, rootView.intrinsicContentSize.width, rootView.intrinsicContentSize.height)];