Clean up EffectiveViewportChanged earlier.

Clean up the `EffectiveViewportChanged` subscriptions before calling `OnDetachedFromVisualTree` so that (un)subscribing to the `EffectiveViewportChanged`  event in `OnDetachedFromVisualTree` doesn't cause a leak.

src/Avalonia.Layout/Layoutable.cs

@@ -758,8 +758,6 @@ namespace Avalonia.Layout
 
         protected override void OnDetachedFromVisualTreeCore(VisualTreeAttachmentEventArgs e)
         {
-            base.OnDetachedFromVisualTreeCore(e);
-
             if (e.Root is ILayoutRoot r)
             {
                 if (_layoutUpdated is object)
@@ -772,6 +770,8 @@ namespace Avalonia.Layout
                     r.LayoutManager.UnregisterEffectiveViewportListener(this);
                 }
             }
+
+            base.OnDetachedFromVisualTreeCore(e);
         }
 
         /// <summary>