Avalonia.Utilities.WeakEvent: null reference exception

Describe the bug Sometimes I get NullReferenceException when using Avalonia with ReactiveUI. Stacktraces:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Avalonia.Utilities.WeakEvent`2.Subscription.OnEvent(Object sender, TEventArgs eventArgs) in /_/src/Avalonia.Base/Utilities/WeakEvent.cs:line 162
   at Avalonia.Utilities.WeakEvents.<>c__DisplayClass5_1.<.cctor>b__7(Object _, PropertyChangedEventArgs e) in /_/src/Avalonia.Base/Utilities/WeakEvents.cs:line 29
   at ReactiveUI.ReactiveObject.ReactiveUI.IReactiveObject.RaisePropertyChanged(PropertyChangedEventArgs args) in /_/src/ReactiveUI/ReactiveObject/ReactiveObject.cs:line 96

System.NullReferenceException: Object reference not set to an instance of an object.
   at Avalonia.Utilities.WeakHashList`1.GetAlive(Func`1 factory) in /_/src/Avalonia.Base/Utilities/WeakHashList.cs:line 210
   at Avalonia.Utilities.WeakEvent`2.Subscription.OnEvent(Object sender, TEventArgs eventArgs) in /_/src/Avalonia.Base/Utilities/WeakEvent.cs:line 157
   at Avalonia.Utilities.WeakEvents.<>c__DisplayClass5_1.<.cctor>b__7(Object _, PropertyChangedEventArgs e) in /_/src/Avalonia.Base/Utilities/WeakEvents.cs:line 29
   at ReactiveUI.ReactiveObject.ReactiveUI.IReactiveObject.RaisePropertyChanged(PropertyChangedEventArgs args) in /_/src/ReactiveUI/ReactiveObject/ReactiveObject.cs:line 96

Full stacktrace

System.NullReferenceException: Object reference not set to an instance of an object.
 at Avalonia.Utilities.WeakHashList`1.GetAlive(Func`1 factory) in /_/src/Avalonia.Base/Utilities/WeakHashList.cs:line 210
 at Avalonia.Utilities.WeakEvent`2.Subscription.OnEvent(Object sender, TEventArgs eventArgs) in /_/src/Avalonia.Base/Utilities/WeakEvent.cs:line 157
 at Avalonia.Utilities.WeakEvents.<>c__DisplayClass5_1.<.cctor>b__7(Object _, PropertyChangedEventArgs e) in /_/src/Avalonia.Base/Utilities/WeakEvents.cs:line 29
 at ReactiveUI.ReactiveObject.ReactiveUI.IReactiveObject.RaisePropertyChanged(PropertyChangedEventArgs args) in /_/src/ReactiveUI/ReactiveObject/ReactiveObject.cs:line 96
 at System.Reactive.AnonymousSafeObserver`1.OnNext(T value) in /_/Rx.NET/Source/src/System.Reactive/AnonymousSafeObserver.cs:line 43
 at System.Reactive.Sink`1.ForwardOnNext(TTarget value) in /_/Rx.NET/Source/src/System.Reactive/Internal/Sink.cs:line 49
 at System.Reactive.Linq.ObservableImpl.SelectMany`2.EnumerableSelector._.OnNext(TSource value) in /_/Rx.NET/Source/src/System.Reactive/Linq/Observable/SelectMany.cs:line 1375
 at System.Reactive.Sink`1.ForwardOnNext(TTarget value) in /_/Rx.NET/Source/src/System.Reactive/Internal/Sink.cs:line 49
 at System.Reactive.Linq.ObservableImpl.Buffer`2.Boundaries._.BufferClosingObserver.OnNext(TBufferClosing value) in /_/Rx.NET/Source/src/System.Reactive/Linq/Observable/Buffer.cs:line 827
 at System.Reactive.Sink`1.ForwardOnNext(TTarget value) in /_/Rx.NET/Source/src/System.Reactive/Internal/Sink.cs:line 49
 at System.Reactive.Linq.ObservableImpl.Merge`1.Observables._.InnerObserver.OnNext(TSource value) in /_/Rx.NET/Source/src/System.Reactive/Linq/Observable/Merge.cs:line 238
 at System.Reactive.Sink`1.ForwardOnNext(TTarget value) in /_/Rx.NET/Source/src/System.Reactive/Internal/Sink.cs:line 49
 at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value) in /_/Rx.NET/Source/src/System.Reactive/Linq/Observable/Select.cs:line 47
 at System.Reactive.Sink`1.ForwardOnNext(TTarget value) in /_/Rx.NET/Source/src/System.Reactive/Internal/Sink.cs:line 49
 at System.Reactive.Linq.ObservableImpl.Where`1.Predicate._.OnNext(TSource value) in /_/Rx.NET/Source/src/System.Reactive/Linq/Observable/Where.cs:line 54
 at System.Reactive.Subjects.Subject`1.OnNext(T value) in /_/Rx.NET/Source/src/System.Reactive/Subjects/Subject.cs:line 147
 at ReactiveUI.IReactiveObjectExtensions.ExtensionState`1.RaisePropertyChanged(String propertyName) in /_/src/ReactiveUI/ReactiveObject/IReactiveObjectExtensions.cs:line 390
 at ReactiveUI.IReactiveObjectExtensions.RaisingPropertyChanged[TSender](TSender reactiveObject, String propertyName) in /_/src/ReactiveUI/ReactiveObject/IReactiveObjectExtensions.cs:line 245
 at ReactiveUI.IReactiveObjectExtensions.RaiseAndSetIfChanged[TObj,TRet](TObj reactiveObject, TRet& backingField, TRet newValue, String propertyName) in /_/src/ReactiveUI/ReactiveObject/IReactiveObjectExtensions.cs:line 131
 at Avalonia.NETCoreMVVMApp2.ViewModels.TreeViewModel.set_IsLoading(Boolean value) in C:\Users\mitya\RiderProjects\Avalonia.NETCoreMVVMApp2\Avalonia.NETCoreMVVMApp2\ViewModels\MainWindowViewModel.cs:line 51
 at Avalonia.NETCoreMVVMApp2.ViewModels.TreeViewModel.Populate(String arg) in C:\Users\mitya\RiderProjects\Avalonia.NETCoreMVVMApp2\Avalonia.NETCoreMVVMApp2\ViewModels\MainWindowViewModel.cs:line 64
 at System.Reactive.PlatformServices.ExceptionServicesImpl.Rethrow(Exception exception) in /_/Rx.NET/Source/src/System.Reactive/Internal/ExceptionServicesImpl.cs:line 19
 at System.Reactive.ExceptionHelpers.Throw(Exception exception) in /_/Rx.NET/Source/src/System.Reactive/Internal/ExceptionServices.cs:line 16
 at System.Reactive.Stubs.<>c.<.cctor>b__2_1(Exception ex) in /_/Rx.NET/Source/src/System.Reactive/Internal/Stubs.cs:line 16
 at System.Reactive.AnonymousSafeObserver`1.OnError(Exception error) in /_/Rx.NET/Source/src/System.Reactive/AnonymousSafeObserver.cs:line 62
 at System.Reactive.Sink`1.ForwardOnError(Exception error) in /_/Rx.NET/Source/src/System.Reactive/Internal/Sink.cs:line 60
 at System.Reactive.ObserveOnObserverNew`1.DrainStep(ConcurrentQueue`1 q) in /_/Rx.NET/Source/src/System.Reactive/Internal/ScheduledObserver.cs:line 553
 at System.Reactive.ObserveOnObserverNew`1.DrainShortRunning(IScheduler recursiveScheduler) in /_/Rx.NET/Source/src/System.Reactive/Internal/ScheduledObserver.cs:line 507
 at System.Reactive.ObserveOnObserverNew`1.<>c.<.cctor>b__17_0(IScheduler scheduler, ObserveOnObserverNew`1 self) in /_/Rx.NET/Source/src/System.Reactive/Internal/ScheduledObserver.cs:line 497
 at Avalonia.ReactiveUI.AvaloniaScheduler.<>c__DisplayClass4_1`1.<Schedule>b__1() in /_/src/Avalonia.ReactiveUI/AvaloniaScheduler.cs:line 48
 at Avalonia.Threading.JobRunner.Job.Avalonia.Threading.JobRunner.IJob.Run() in /_/src/Avalonia.Base/Threading/JobRunner.cs:line 193
 at Avalonia.Threading.JobRunner.RunJobs(Nullable`1 priority) in /_/src/Avalonia.Base/Threading/JobRunner.cs:line 38
 at Avalonia.Win32.Win32Platform.WndProc(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam) in /_/src/Windows/Avalonia.Win32/Win32Platform.cs:line 257
 at Avalonia.Win32.Interop.UnmanagedMethods.DispatchMessage(MSG& lpmsg)
 at Avalonia.Win32.Win32Platform.RunLoop(CancellationToken cancellationToken) in /_/src/Windows/Avalonia.Win32/Win32Platform.cs:line 204
 at Avalonia.Threading.Dispatcher.MainLoop(CancellationToken cancellationToken) in /_/src/Avalonia.Base/Threading/Dispatcher.cs:line 61
 at Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.Start(String[] args) in /_/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs:line 120
 at Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime(AppBuilder builder, String[] args, ShutdownMode shutdownMode) in /_/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs:line 212
 at Avalonia.NETCoreMVVMApp2.Program.Main(String[] args) in C:\Users\mitya\RiderProjects\Avalonia.NETCoreMVVMApp2\Avalonia.NETCoreMVVMApp2\Program.cs:line 17

I have two TreeView controls with following viewmodels. After Task.Delay in Populate method is finished and IsLoading is set to false exception is thrown

public class TreeViewModel : ReactiveObject
{
    public TreeViewModel()
    {
        this.WhenAnyValue(x => x.Filter)
            .Throttle(TimeSpan.FromMilliseconds(800))
            .Select(text => text.Trim())
            .DistinctUntilChanged()
            .SelectMany(Populate)
            .ObserveOn(RxApp.MainThreadScheduler)
            .Subscribe();
    }

    public ObservableCollection<NodeViewModel> Items { get; } = new();

    [Reactive] public bool IsLoading { get; set; }

    [Reactive] public string Filter { get; set; } = string.Empty;

    private async Task<Unit> Populate(string arg)
    {
        try
        {
            IsLoading = true;
            await Task.Delay(2000);
        }
        finally
        {
            IsLoading = false;
        }

        return Unit.Default;
    }

}

<TextBlock Text="Loading..." IsVisible="{Binding Tree1.IsLoading}" />
<TreeView Items="{Binding Tree1.Items}" >
  <TreeView.Styles>
    <Style Selector="TreeViewItem" x:DataType="viewModels:NodeViewModel">
      <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
      <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
    </Style>
  </TreeView.Styles>
  <TreeView.DataTemplates>
    <TreeDataTemplate DataType="viewModels:NodeViewModel" ItemsSource="{Binding Children}">
      <TextBlock Text="{Binding Value}" />
    </TreeDataTemplate>
  </TreeView.DataTemplates>
</TreeView>

To Reproduce Steps to reproduce the behavior:

1. Run attached repro project
2. See the error (maybe you have to launch it several times)

Expected behavior No exception is thrown

Desktop (please complete the following information):

• OS: [Windows]
• Version [11-preview4, 11-preview5, .net6, .net7]

Additional context Version 0.10.18 worked without errors.

Repro Avalonia.NETCoreMVVMApp2.zip

[maxkatz6]

In general it looks like a duplicate of this issue https://github.com/AvaloniaUI/Avalonia/issues/8810 With more details from @grokys .

[maxkatz6]

Closing as a duplicate of https://github.com/AvaloniaUI/Avalonia/issues/8810