ComboBox set SelectedItem null when close the app,binding issue maybe?

[SharpDent]

when I choose this option and close the app,the selectedItem will be set null,how come?

ViewModel

using System.Collections.ObjectModel;
using CommunityToolkit.Mvvm.ComponentModel;

namespace Avalonia.ComboBoxBinding.ViewModels;

public partial class MainViewModel : ObservableObject
{
    [ObservableProperty] private Person? _selectedPerson;

    public ObservableCollection<Person> People { get; set; } =
    [
        new()
        {
            Name = "John",
            Age = 25,
        },
        new()
        {
            Name = "Jane",
            Age = 25,
        },
        new()
        {
            Name = "Mary",
            Age = 25,
        }
    ];

    public ObservableCollection<Teacher> Teachers { get; set; } =
    [
        new()
        {
            Name = "Aohn",
            Age = 25,
        },
        new()
        {
            Name = "Aane",
            Age = 25,
        },
        new()
        {
            Name = "Aary",
            Age = 25,
        }
    ];
}

public partial class Teacher : ObservableObject
{
    [ObservableProperty] private string _name = string.Empty;
    [ObservableProperty] private int _age;
}

public partial class Person : ObservableObject
{
    [ObservableProperty] private string _name = string.Empty;
    [ObservableProperty] private int _age;
    private Teacher? _teacher;

    public Teacher? Teacher
    {
        get => _teacher;
        set
        {
            if (value is null) Console.WriteLine("value is null");
            SetProperty(ref _teacher, value);
        }
    }
}

View

<Window
    Title="MainWindow"
    d:DesignHeight="450"
    d:DesignWidth="800"
    mc:Ignorable="d"
    x:Class="Avalonia.ComboBoxBinding.Views.MainWindow"
    xmlns="https://github.com/avaloniaui"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:viewModels="using:Avalonia.ComboBoxBinding.ViewModels"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Design.DataContext>
        <viewModels:MainViewModel />
    </Design.DataContext>
    <DockPanel>
        <ListBox
            DockPanel.Dock="Left"
            ItemsSource="{Binding People}"
            SelectedItem="{Binding SelectedPerson}"
            Width="150">
            <ListBox.DataTemplates>
                <DataTemplate DataType="viewModels:Person">
                    <TextBlock Text="{Binding Name}" />
                </DataTemplate>
            </ListBox.DataTemplates>
        </ListBox>
        <ContentControl Content="{Binding SelectedPerson}">
            <ContentControl.DataTemplates>
                <DataTemplate DataType="viewModels:Person">
                    <StackPanel Margin="10">
                        <TextBlock Text="{Binding Name}" />
                        <TextBlock Text="{Binding Age}" />
                        <ComboBox ItemsSource="{Binding $parent[Window].DataContext.Teachers}" SelectedItem="{Binding Teacher}">
                            <ComboBox.DataTemplates>
                                <DataTemplate DataType="viewModels:Teacher">
                                    <TextBlock Text="{Binding Name}" />
                                </DataTemplate>
                            </ComboBox.DataTemplates>
                        </ComboBox>
                    </StackPanel>
                </DataTemplate>
            </ContentControl.DataTemplates>
        </ContentControl>
    </DockPanel>
</Window>