Avalonia notification causes the application to crash

Question

I'm testing Avalonia notifications (C#, Windows 10, Visual Studio Community 2019). The following code crashes whenever I push the button and change the property 'Greeting'.

I suppose the problem is what the remarks indicate in the picture. The question is how to avoid that? How to make a notification appear after I change the content of the property Greeting? Here is the code:

using Avalonia;
using Avalonia.Controls;
using Avalonia.Interactivity;
using Avalonia.Markup.Xaml;
using Avalonia.Controls.Notifications;
using System;

namespace Avalonia_test_Notifications.Views
{
    public partial class MainWindow : Window
    {
        public ViewModels.MainWindowViewModel ViewModel;
        public WindowNotificationManager Manager;
        public MainWindow()
        {            
            InitializeComponent();
            this.DataContext = ViewModel;
#if DEBUG
            this.AttachDevTools();
#endif
    }


    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load(this);
        Manager = new WindowNotificationManager(this);
        ViewModel = new ViewModels.MainWindowViewModel(Manager);
    }

    private void ShowNotification(object sender, RoutedEventArgs e)
    {
        ViewModel.ClickEvent(sender, e);
    
    }

    
}
}

and

using System;
using System.Collections.Generic;
using System.Text;
using ReactiveUI;
using System.Reactive;

namespace Avalonia_test_Notifications.ViewModels
{
    public class MainWindowViewModel : ViewModelBase
    {
        private string _greeting = "Greetings";
        public string Greeting
        {
            get => _greeting;
            set 
            { 
                this.RaiseAndSetIfChanged(ref _greeting, value); 
            }
        }
        public Avalonia.Controls.Notifications.Notification Notification;
        public Avalonia.Controls.Notifications.IManagedNotificationManager Manager;
        public string [] stringArray;
        public MainWindowViewModel(Avalonia.Controls.Notifications.IManagedNotificationManager manager)
        {
            stringArray = new string[3] { "message 1", "message 2", "message 3" };
            Manager = manager;            
            this.WhenAnyValue(x => x.Greeting).Subscribe(x=>ShowNotification(x));
        }

        public void ClickEvent(object sender, Avalonia.Interactivity.RoutedEventArgs e)
        {
            System.Random rand = new System.Random();
            int x = rand.Next(0, 3);
            Greeting = stringArray[x];
        }
        
        public void ShowNotification(string s)
        {
            Notification = new Avalonia.Controls.Notifications.Notification("TestNotification", s, Avalonia.Controls.Notifications.NotificationType.Information, TimeSpan.FromSeconds(0));
            Manager.Show(Notification);
        }
    }
}

and finally:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:Avalonia_test_Notifications.ViewModels"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="Avalonia_test_Notifications.Views.MainWindow"
        Icon="/Assets/avalonia-logo.ico"
        Title="Avalonia_test_Notifications">

    <Design.DataContext>
        <vm:MainWindowViewModel/>
    </Design.DataContext>
  <StackPanel Orientation="Vertical">
    <TextBlock Text="{Binding Greeting}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
    <Button Content="Notification" Click="ShowNotification"></Button>
</StackPanel>
</Window>

Answer 1

It seems that the MainWindow needs some time to prepare before interacting with it. The solution I've found is to respond to changes only when IsActive is true. I'm passing a reference to the MainWindow when creating the ViewModel instead of passing a reference to the NotificationManager.

 public MainWindowViewModel(Views.MainWindow Window)
        {
            stringArray = new string[3] { "message 1", "message 2", "message 3" };
            Manager = new Avalonia.Controls.Notifications.WindowNotificationManager(Window);
            
            this.WhenAnyValue(x => x.Greeting).WhereNotNull()
                .Subscribe(x=>
            {
                if (Window.IsActive)
                ShowNotification(x);
            }
            );
        }