(More documentation for this node is coming)
This value can be null.
The UIKit.UIBarButtonItems are displayed right-to-left. In other words, the UIKit.UIBarButtonItem at index 0 is the right-most displayed, the one at index 1 is displayed to the left of that, etc.
UIKit.UIBarButtonItems that would overlap the UINavigationItem.TItleView are not displayed.