WeakHashList Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.

Describe the bug

Unhandled exception. 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 CommunityToolkit.Mvvm.ComponentModel.ObservableObject.OnPropertyChanged(PropertyChangedEventArgs e)
   at CommunityToolkit.Mvvm.ComponentModel.ObservableObject.OnPropertyChanged(String propertyName)
   at CommunityToolkit.Mvvm.ComponentModel.ObservableObject.SetProperty[T](T& field, T newValue, String propertyName)
   at SvgToXaml.ViewModels.FileItemViewModel.set_Svg(SvgViewModel value) in /Users/wieslawsoltes/Documents/GitHub/SvgToXaml/src/SvgToXaml.Base/ViewModels/FileItemViewModel.cs:line 37
   at SvgToXaml.ViewModels.FileItemViewModel.<>c__DisplayClass28_0.<Load>b__0() in /Users/wieslawsoltes/Documents/GitHub/SvgToXaml/src/SvgToXaml.Base/ViewModels/FileItemViewModel.cs:line 86
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location ---
   at SvgToXaml.ViewModels.FileItemViewModel.Load(DrawAttributes ignoreAttribute) in /Users/wieslawsoltes/Documents/GitHub/SvgToXaml/src/SvgToXaml.Base/ViewModels/FileItemViewModel.cs:line 84
   at SvgToXaml.ViewModels.MainWindowViewModel.<.ctor>b__31_9(FileItemViewModel x) in /Users/wieslawsoltes/Documents/GitHub/SvgToXaml/src/SvgToXaml.Base/ViewModels/MainWindowViewModel.cs:line 93
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_0(Object state)
   at Avalonia.Threading.JobRunner.RunJobs(Nullable`1 priority) in /_/src/Avalonia.Base/Threading/JobRunner.cs:line 38
   at Avalonia.Native.PlatformThreadingInterface.SignaledCallback.Signaled(Int32 priority, Int32 priorityContainsMeaningfulValue) in /_/src/Avalonia.Native/PlatformThreadingInterface.cs:line 39
   at Avalonia.Native.Interop.Impl.__MicroComIAvnSignaledCallbackVTable.Signaled(Void* this, Int32 priority, Int32 priorityContainsMeaningfulValue) in /_/src/Avalonia.Native/Interop.Generated.cs:line 4502
--- End of stack trace from previous location ---
   at Avalonia.Native.PlatformThreadingInterface.RunLoop(CancellationToken cancellationToken) in /_/src/Avalonia.Native/PlatformThreadingInterface.cs:line 90
   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[T](T builder, String[] args, ShutdownMode shutdownMode) in /_/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs:line 209
   at SvgToXaml.Program.Main(String[] args) in /Users/wieslawsoltes/Documents/GitHub/SvgToXaml/src/SvgToXaml.Desktop/Program.cs:line 14

To Reproduce

Run SvgToXaml.Desktop app and add some items, change selections in left ListBox, exception sometimes happens.

Expected behavior

No exception

Screenshots

Desktop (please complete the following information):

• OS: macOS M1
• Version 11.0.0-preview1

Additional context Add any other context about the problem here.

[grokys]

Thanks @sdijoseph !

[batzen]

This also happens in my application. In my case i have instances of ObservableCollection which are updated on background threads, but those updates are protected by locks so it’s in fact thread safe. With Avalonia 10 this wasn’t an issue, but with 11 it just throws an NRE.

The exception is not always thrown in WeakHashList but sometimes later in Subscription.OnEvent and then in foreach(var item in alive.Span) where alive is from WeakHashList.GetAlive().