issue-829: A recomendation to better access the NavigationBar in NavigationItem

Sources/iOS/NavigationItem.swift

@@ -84,8 +84,16 @@ public class NavigationItem: NSObject {
         }
     }
 	
+    /// An optional reference to the NavigationBar.
     public var navigationBar: NavigationBar? {
-        return contentView.superview?.superview as? NavigationBar
+        var v = contentView.superview
+        while nil != v {
+            if let navigationBar = v as? NavigationBar {
+                return navigationBar
+            }
+            v = v?.superview
+        }
+        return nil
     }
     
     open override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {