The Messenger can't send Message by using the official sample from "MVVM Toolkit Sample App" software

[wxysy]

Describe the bug

When I learn the Messenger part of CommunityToolkit.Mvvm by following the official sample from "MVVM Toolkit Sample App" software, I find that the sample code doesn't work when I use them in my program.

Regression

No response

Steps to reproduce

1.Condition
Framework: .Net 6 WPF 
      IDE: VisualStudio Professional 17.4.2;
 Language: C#

2.Code
The C# Code:

/// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    [ObservableObject]
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            //UserReceiver = new UserReceiverViewModel();
            //UserSender = new UserSenderViewModel();
        }

        public UserReceiverViewModel UserReceiver { get; } = new();
        public UserSenderViewModel UserSender { get; } = new();

        private void Btn_SendMes_Click(object sender, RoutedEventArgs e)
        {
            UserSender.SendUserMessage();
            tb_Show.Text += "信息已发送\n";
            tb_Show.Text += $"接收的信息：{UserReceiver.MyStringReceive}\n";
        }
    }

/// <summary>
    /// 用于SendingMessage的信息类
    /// </summary>
    /// <typeparam name="T">该类承载的信息类型</typeparam>
    public sealed class SendingMessageCarrier<T> : ValueChangedMessage<T>
    {
        public SendingMessageCarrier(T value) : base(value)
        {
        }
    }

public class UserSenderViewModel : ObservableRecipient
    {
        public string MyString { get; set; } = "wxy";

        public void SendUserMessage()
        {
            var carrier = new SendingMessageCarrier<string>(MyString);
            Messenger.Send(carrier);
            //WeakReferenceMessenger.Default.Send(carrier);
            //WeakReferenceMessenger.Default.Send(carrier, 12);

            if (MyString == "wxy")
            {
                MyString = "zbk";
            }
            else
            {
                MyString = "wxy";
            }
        }
    }

public class UserReceiverViewModel : ObservableRecipient
    {
        public string MyStringReceive { get; set; } = string.Empty;

        protected override void OnActivated()
        {
            //base.OnActivated();
            Messenger.Register<UserReceiverViewModel, SendingMessageCarrier<string>>(this, MessageHandler);
            //WeakReferenceMessenger.Default.Register<UserReceiverViewModel, SendingMessageCarrier<string>>(this, MessageHandler);
            //WeakReferenceMessenger.Default.Register<UserReceiverViewModel, SendingMessageCarrier<string>, int>(this, 12, MessageHandler);
        }

        private void MessageHandler(UserReceiverViewModel recipient, SendingMessageCarrier<string> message)
        {
            recipient.MyStringReceive = message.Value;
        }
    }


The XAML Code
```xaml
<Window x:Class="LearnMessenger.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:LearnMessenger"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <TextBox x:Name="textBox" HorizontalAlignment="Left" Margin="10,10,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="385" FontSize="18" IsReadOnly="True" Height="207"/>
        <TextBox x:Name="tb_Show" Margin="400,10,10,0" TextWrapping="Wrap" VerticalAlignment="Top" FontSize="18" IsReadOnly="True" Height="207"/>
        <Button x:Name="btn_SendMes" Click="Btn_SendMes_Click" Content="SendMessage" HorizontalAlignment="Left" Margin="24,365,0,0" VerticalAlignment="Top" Height="46" Width="143"/>
    </Grid>
</Window>

3.Click on Button Control(active the method ) 4.the UserReceiverViewModel instance cann't receive message from UserSenderViewModel instance.

### Expected behavior

I want to know why the code doesn't work and tell me how to fix it.

### Screenshots

_No response_

### IDE and version

VS 2022

### IDE version

17.4.2

### Nuget packages

- [ ] CommunityToolkit.Common
- [ ] CommunityToolkit.Diagnostics
- [ ] CommunityToolkit.HighPerformance
- [X] CommunityToolkit.Mvvm (aka MVVM Toolkit)

### Nuget package version(s)

8.0.0

### Additional context

_No response_

### Help us help you

Yes, I'd like to be assigned to work on this item

[faceoffers28]

I got this working on Android using .Net 7 targeting API 33 running on Android 31. It's even working when called from a foreground service. It's working on iOS using .Net 7.0 targeting iOS 16, but only when called from a page behind or viewmodel. If I call it from a service, it does not work. There is no error being reported in the try catch. Is it supposed to work from AppDelegate, a service etc... in iOS??? I'm using version 1.3.0 of the toolkit. I'm going to upgrade and see what happens.

I used this video to setup the pub/sub messenger. https://www.youtube.com/watch?v=vD17OetzGXc

I just upgraded to Maui Toolkit 3.0.0. It doesn't send a message from AppDelegate, a service etc.... Is it supposed to???