Include ApplicationUser returns null - asp.net

Question

I'm trying to include ApplicationUser, and BoardEntityFiles in BoardEntity.
When I try to include the ApplicationUser, it fetches nothing.
The structure is
BoardEntity has many BoardEntityFiles, And ApplicationUser has many BoardEntity.
using Microsoft.AspNetCore.Identity;
namespace Tutorial1.Entity;
public class ApplicationUser : IdentityUser
{
public String FirstName { get; set; }
public String LastName { get; set; }
}
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
namespace Tutorial1.Entity;
public class BoardEntity
{
public int? Id { get; set; }
public string? ApplicationUserId { get; set; }
[ForeignKey("ApplicationUserId")]
public ApplicationUser? ApplicationUser { get; set; }
[Required]
[StringLength(255, MinimumLength = 1)]
public string Title { get; set; }
[StringLength(2000, MinimumLength = 1)]
public string? Content { get; set; }
public DateTime? CreatedAt { get; set; }=DateTime.UtcNow;
public DateTime? UpdatedAt { get; set; }
public List<BoardFileEntity>? BoardFiles { get; set; }
}
using System.ComponentModel.DataAnnotations.Schema;
namespace Tutorial1.Entity;
public class BoardFileEntity
{
public int? Id { get; set; }
public string? FilePath { get; set; }
public string? EncName { get; set; }
public string? OriginalName { get; set; }
public string ? Mimetype { get; set; }
public string? Extension { get; set; }
public int? BoardEntityId { get; set; }
[ForeignKey("BoardEntityId")]
public BoardEntity BoardEntity { get; set; }
}
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.EntityFrameworkCore;
using Tutorial1.Data;
using Tutorial1.Entity;
namespace Tutorial1.Pages.FileUploads;
public class IndexModel : PageModel
{
private readonly ApplicationDbContext _db;
public IList<BoardEntity> BoardEntity { get; set; }
public IndexModel(ApplicationDbContext db)
{
_db = db;
}
public async Task OnGet()
{
BoardEntity = await _db.BoardEntities.Include(e=>e.BoardFiles).Include(e=>e.ApplicationUser).ToListAsync();
}
}
BoardEntity = await _db.BoardEntities.Include(e=>e.BoardFiles).Include(e=>e.ApplicationUser).ToListAsync();
When I execute this,BoardEntity.BoardFiles are fetched nicely, but BoardEntity.ApplicationUser is null.