fix flaky top bar transition

lib/ios/RNNRootViewController.m

 		} if ([self.navigationItem.title isKindOfClass:[RNNCustomTitleView class]] && !_customTitleView) {
 			self.navigationItem.title = nil;
 		}
-    } else if (_customTitleView.superview == nil) {
+    } else if (_customTitleView && _customTitleView.superview == nil) {
         if ([self.navigationItem.title isKindOfClass:[RNNCustomTitleView class]] && !_customTitleView) {
             self.navigationItem.title = nil;
         }
 		} else if ([[self.navigationController.navigationBar.subviews lastObject] isKindOfClass:[RNNCustomTitleView class]] && !_customTopBar) {
 			[[self.navigationController.navigationBar.subviews lastObject] removeFromSuperview];
 		}
-    } else if (_customTopBar.superview == nil) {
+    } else if (_customTopBar && _customTopBar.superview == nil) {
         if ([[self.navigationController.navigationBar.subviews lastObject] isKindOfClass:[RNNCustomTitleView class]] && !_customTopBar) {
             [[self.navigationController.navigationBar.subviews lastObject] removeFromSuperview];
         }
 			[[self.navigationController.navigationBar.subviews objectAtIndex:1] removeFromSuperview];
 			self.navigationController.navigationBar.clipsToBounds = NO;
 		}
-    } if (_customTopBarBackground.superview == nil) {
+    } if (_customTopBarBackground && _customTopBarBackground.superview == nil) {
         if ([[self.navigationController.navigationBar.subviews objectAtIndex:1] isKindOfClass:[RNNCustomTitleView class]]) {
             [[self.navigationController.navigationBar.subviews objectAtIndex:1] removeFromSuperview];
         }