The default value is null.
This value can be null.
If this method returns the default null value, the UIKit.UIView will not have its state preserved and restored.
If the UIKit.UIView does need state preservation and restoration, this method should be overridden to return a string.
In addition to returning a non-null UIView.RestorationIdentifier, so too must the UIKit.UIViewController for which this UIKit.UIView is the UIViewController.View and all the ancestor UIKit.UIViewControllers reachable by the UIViewController.Parent property.