Avoid unnecessary allocations on DependencyObjectExtensions.RegisterD…

…isposableNestedPropertyChangedCallback (#705)

src/Uno.Toolkit.UI/Controls/NavigationBar/NavigationBarNavigationItemRenderer.iOS.cs

@@ -231,7 +231,7 @@ public override void SetSuperviewNeedsLayout()
 		// Even though we set the Navigation as the parent of the TitleView,
 		// it will change to the native control when the view is added (once MovedToSuperview is called).
 		// This native control is the visual parent but is not a DependencyObject and will not propagate the DataContext.
-		// In order to ensure the DataContext is propagated properly, we need to notify the renderer that this change has occured
+		// In order to ensure the DataContext is propagated properly, we need to notify the renderer that this change has occurred
 		// so we can restore the NavigationBar parent that can propagate the DataContext
 		public override void MovedToSuperview()
 		{

src/Uno.Toolkit.UI/Extensions/DependencyObjectExtensions.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using Uno.Disposables;
@@ -53,12 +53,11 @@ internal static IDisposable RegisterDisposableNestedPropertyChangedCallback(this
 				.Select(group =>
 				{
 					var property = group.Key;
-					var subProperties = group.ToArray();
-
-					var childDisposable = new SerialDisposable();
 
 					if (instance.TryGetValue(property, out var dpValue))
 					{
+						var childDisposable = new SerialDisposable();
+						var subProperties = group.ToArray();
 						childDisposable.Disposable = dpValue?.RegisterDisposableNestedPropertyChangedCallback(callback, subProperties);
 
 						var disposable = instance.RegisterDisposablePropertyChangedCallback(property, (s, e) =>