音声再生時にNullReferenceExceptionが発生する

[indigo-san]

該当するコード
https://github.com/b-editor/beutl/blob/main/src%2FBeutl%2FViewModels%2FPlayerViewModel.cs#L268-L269

スタックトレース

System.NullReferenceException: Object reference not set to an instance of an object.
   at Reactive.Bindings.ReactivePropertySlim`1.Subscribe(IObserver`1 observer)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
--- End of stack trace from previous location ---
   at System.Reactive.PlatformServices.ExceptionServicesImpl.Rethrow(Exception exception)
   at System.Reactive.ExceptionHelpers.Throw(Exception exception)
   at System.Reactive.Stubs.<>c.<.cctor>b__2_1(Exception ex)
   at System.Reactive.AnonymousSafeObserver`1.OnError(Exception error)
   at System.Reactive.Sink`1.ForwardOnError(Exception error)
   at System.Reactive.Sink`2.OnError(Exception error)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Sink`2.Run(IObservable`1 source)
   at System.Reactive.Linq.ObservableImpl.Where`1.Predicate.Run(_ sink)
   at System.Reactive.Concurrency.CurrentThreadScheduler.Schedule[TState](TState state, TimeSpan dueTime, Func`3 action)
   at System.Reactive.Concurrency.Scheduler.ScheduleAction[TState](IScheduler scheduler, TState state, Action`1 action)
   at System.Reactive.Producer`2.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at Beutl.ViewModels.PlayerViewModel.PlayWithXA2(XAudioContext audioContext, Scene scene)
   at Beutl.ViewModels.PlayerViewModel.PlayAudio(Scene scene)

バージョン:
1.0.0-preview.3