Added missing setters.

src/Avalonia.Controls/Window.cs

@@ -82,7 +82,10 @@ namespace Avalonia.Controls
         /// Defines the <see cref="WindowStartupLocation"/> proeprty.
         /// </summary>
         public static readonly DirectProperty<Window, WindowStartupLocation> WindowStartupLocationProperty =
-            AvaloniaProperty.RegisterDirect<Window, WindowStartupLocation>(nameof(WindowStartupLocation), o => o.WindowStartupLocation);
+            AvaloniaProperty.RegisterDirect<Window, WindowStartupLocation>(
+                nameof(WindowStartupLocation),
+                o => o.WindowStartupLocation,
+                (o, v) => o.WindowStartupLocation = v);
 
         private readonly NameScope _nameScope = new NameScope();
         private object _dialogResult;

src/Avalonia.Controls/WindowBase.cs

@@ -33,7 +33,10 @@ namespace Avalonia.Controls
         /// Defines the <see cref="Owner"/> property.
         /// </summary>
         public static readonly DirectProperty<WindowBase, WindowBase> OwnerProperty =
-            AvaloniaProperty.RegisterDirect<WindowBase, WindowBase>(nameof(Owner), o => o.Owner);
+            AvaloniaProperty.RegisterDirect<WindowBase, WindowBase>(
+                nameof(Owner),
+                o => o.Owner,
+                (o, v) => o.Owner = v);
 
         private bool _hasExecutedInitialLayoutPass;
         private bool _isActive;