Instance can be null here.

Was causing intermittently failing test: https://ci.appveyor.com/project/AvaloniaUI/Avalonia/build/0.1.4468

src/Markup/Avalonia.Markup/Data/Plugins/AvaloniaPropertyAccessorPlugin.cs

@@ -104,7 +104,7 @@ namespace Avalonia.Markup.Data.Plugins
 
             protected override void SubscribeCore(IObserver<object> observer)
             {
-                _subscription = Instance.GetWeakObservable(_property).Subscribe(observer);
+                _subscription = Instance?.GetWeakObservable(_property).Subscribe(observer);
             }
         }
     }