UWP - How to pass in a non null XamlRoot of a page to a NavigationService called by ViewModel associated to the page?

In my UWP app MVVM implementation, my ViewModels do not know about their View.

My ViewModels call a NavigationService.DisplayAlert() method. I need the ContentDialog to be shown from the AppWindow of the page associated with the ViewModel.

To do so, I follow that documentation. I try to pass in the AppWindow XamlRoot to the ViewModel. However, whatever UIElement of the Page used for the AppWindow I take, it has a null XamlRoot. How come?

How can I pass in my AppWindow XamlRoot to its ViewModel?

In my NavigationService:

public async Task DisplayAlert(string message, object existingElementXamlRoot)
{
    var messageDialog = new ContentDialog
    {
        Title = "Message",
        Content = message,
        CloseButtonText = "Ok"
    };
    if (ApiInformation.IsApiContractPresent("Windows.Foundation.UniversalApiContract", 8) && existingElementXamlRoot is XamlRoot xamlRoot) messageDialog.XamlRoot = xamlRoot;
    await messageDialog .ShowAsync();
}

In my BasePage:

protected override void OnNavigatedTo(NavigationEventArgs e)
{
    DataContext = e.Parameter;
    ((IBasePageViewModel) DataContext).XamlRoot = this.Content.XamlRoot;//I cannot find any UIElement with a non null XamlRoot
    base.OnNavigatedTo(e);
}