❔ please help fix NullRefException

public partial class MainPage : ContentPage, INotifyPropertyChanged
{
    private List<Contact> _contacts;
    public List<Contact> Contacts { get => _contacts; set { _contacts = value; PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Contacts")); } }
    public event PropertyChangedEventHandler PropertyChanged;
    public MainPage()
    {
        InitializeComponent();
        BindingContext = this;
    }
    private void Button_Clicked(object sender, EventArgs e)
    {
        Contacts = new List<Contact>
        {
            new Contact(){ Age = "24" }
        };
    }
}

<VerticalStackLayout>
        <Button Text="ClickMe" Clicked="Button_Clicked"/>
        <ListView ItemsSource="{Binding Contacts}">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <Label TextColor="White" BackgroundColor="Green" Text="{Binding Age}"/>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
</VerticalStackLayout>