The default value is null.
This value can be null.
If this method returns the default null value, the MonoTouch.UIKit.UIView will not have its state preserved and restored.
If the MonoTouch.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 MonoTouch.UIKit.UIViewController for which this MonoTouch.UIKit.UIView is the UIViewController.View and all the ancestor MonoTouch.UIKit.UIViewControllers reachable by the UIViewController.Parent property.