NullReferenceException on a not null value in C#

Question

I am getting a NullReferenceException on an object which is not null I have a model of name articles in my entity framework db. Below is the model class

public class Articles
{
     [Key]
     public int Id { get; set; }
    
     [Required(ErrorMessage = "Title is needed")]
     public string Title { get; set; }
    
     [Required(ErrorMessage = "Author is needed")]
     public ApplicationUser Author { get; set; }
    
     [Required(ErrorMessage = "Content is needed")]
     [Display(Name ="Content")]
     public string Content { get; set; }

     [Required(ErrorMessage = "Subject is needed")]
     public string Subject { get; set; }
}

Below is the action where I am trying to retrieve the Author

[HttpGet]
public async Task<IActionResult> ListArticles() 
{
     var query = from a in this.context.articles select a;
     string id = string.Empty;
     foreach (var q in query) 
     {
          id = q.Author.ToString();//THIS WHERE I AM GETTING NullReferenceException

          return View(query);
     }
}

Here is the view snippet for this controller action

@foreach(var m in Model)
{
     <div class="card-body">
         <div class="row">
             <div class="col">
                 <a href="#" class="nav-link">@m.Title</a>
             </div>
         </div>
         <div class="row">
             <div class="col-md-6">By: @user</div>
                 <div class="col-md-6">Subject: @m.Subject</div>
             </div>
         </div>
     </div>
}

Snapshot of my article model

PS: Why did the Author field turned to AuthorId in the model?

Here is the ApplicationUser class (Type of Author in the model class)

public class ApplicationUser : IdentityUser
{
     public string Firstname { get; set; }
     public string Lastname { get; set; }
     public bool isUser { get; set; }
     public bool isAuthor { get; set; }
     public bool isAdmin { get; set; }
}

Forgive me If there is a silly mistake I am very new to asp.net mvc