Don't throw trying to unsub if not subscribed.

This can happen when re-evaluating the value store if a style is detached before its frame is evaluated.

src/Avalonia.Base/Styling/Activators/StyleActivatorBase.cs

@@ -29,6 +29,9 @@ namespace Avalonia.Styling.Activators
 
         public void Unsubscribe(IStyleActivatorSink sink)
         {
+            if (_sink is null)
+                return;
+
             if (_sink != sink)
             {
                 throw new AvaloniaInternalException("StyleActivatorSink is not subscribed.");