Fix copypasta.

Co-authored-by: Max Katz <[email protected]>

src/Avalonia.Controls/Viewbox.cs

@@ -95,7 +95,7 @@ namespace Avalonia.Controls
                     LogicalChildren.Remove(oldChild);
                 }
 
-                _containerVisual.Child = change.GetNewValue<IControl>();
+                _containerVisual.Child = newChild;
 
                 if (newChild is not null)
                 {