Passing model from Index to _Layout to _Header in ASP.NET Core causes NullReferenceException on get

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Index
_Layout
_Header
_Header
get_Model()
@page
@{
Layout = "_Layout";
ViewData["_Header"] = new DemoForStackOverflow.Pages.Shared.Header() {IsArriving = true};
}
h2 some content /h2

@page
@{
Layout = "_Layout";
ViewData["_Header"] = new DemoForStackOverflow.Pages.Shared.Header() {IsArriving = true};
}
h2 some content /h2

!DOCTYPE html
html
head
title Demo for Stack Overflow /title
/head
body
div id="header"
@await Html.PartialAsync("_Header", ViewData["_Header"])
/div
h1 Appears on every page below header and above body /h1
div id="body"
@RenderBody()
/div
/body
/html

!DOCTYPE html
html
head
title Demo for Stack Overflow /title
/head
body
div id="header"
@await Html.PartialAsync("_Header", ViewData["_Header"])
/div
h1 Appears on every page below header and above body /h1
div id="body"
@RenderBody()
/div
/body
/html

@page
@model DemoForStackOverflow.Pages.Shared.Header

@if (Model.IsArriving)
{
p Hello /p
}
else
{
p Goodbye /p
}

@page
@model DemoForStackOverflow.Pages.Shared.Header

@if (Model.IsArriving)
{
p Hello /p
}
else
{
p Goodbye /p
}

using Microsoft.AspNetCore.Mvc.RazorPages;

namespace DemoForStackOverflow.Pages.Shared;

public class Header : PageModel
{
public bool IsArriving { get; set; }
}

using Microsoft.AspNetCore.Mvc.RazorPages;

namespace DemoForStackOverflow.Pages.Shared;

public class Header : PageModel
{
public bool IsArriving { get; set; }
}

Here is the full exception message and stack trace:
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]

An unhandled exception has occurred while executing the request.

System.NullReferenceException: Object reference not set to an instance of an object.

at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage page, ViewContext context)
at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, ViewContext context, Boolean invokeViewStarts)
at Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(ViewContext context)
at Microsoft.AspNetCore.Mvc.ViewFeatures.HtmlHelper.RenderPartialCoreAsync(String partialViewName, Object model, ViewDataDictionary viewData, TextWriter writer)
at Microsoft.AspNetCore.Mvc.ViewFeatures.HtmlHelper.PartialAsync(String partialViewName, Object model, ViewDataDictionary viewData)
at MicrosoSource of the question:

Question and source license information: