Error:-Object reference not set to an instance of an object

I actually wanted to save data to my relational database. But I have fallen down a strange problem when i trying to add the data into database. I don’t understand why I found this issue.
Here is the code:-

Class model:-

 public class AppUser : IdentityUser<int>
    {
       [JsonIgnore]
       public List<QuestionPost> Qpost { get; set; }
       [JsonIgnore]
       public List<Comment> Comments { get; set; }
    }

 public class QuestionPost
    {
        public int Id { get; set; }
        public string Title { get; set; }
        public int? UserId { get; set; }
        public AppUser User { get; set; }
        public List<Comment> Comments { get; set; }
    }

 public class Comment
    {
        public int Id { get; set; }
        public string Content { get; set; }
        public int? UserId { get; set; }
        public AppUser User { get; set; }
        public int? PostId { get; set; }
        public QuestionPost Qpost { get; set; }
    }

Datacontext.cs

...
 builder.Entity<AppUser>()
            .HasMany(u => u.Qpost)
            .WithOne(p => p.User)
            .HasForeignKey(p => p.UserId);

        builder.Entity<AppUser>()
            .HasMany(u => u.Comments)
            .WithOne(c => c.User)
            .HasForeignKey(c => c.UserId);

        builder.Entity<QuestionPost>()
            .HasMany(p => p.Comments)
            .WithOne(c => c.Qpost)
            .HasForeignKey(c => c.PostId);

and there is my controller class:-

        [HttpPost("addq")]
        public async Task<IActionResult> CreateComment( [FromBody]Comment comment)
        {
            var qpost = await _context.QuestionPosts
                .FirstOrDefaultAsync(p => p.Id == comment.PostId);

            var UserId = int.Parse(User.GetUserId());
            
            var user = await _context.Users
                .FirstOrDefaultAsync(u => u.Id == UserId);
           
            if (qpost != null && user != null)
            {
                comment.Qpost = qpost;
                comment.User = user;
                qpost.Comments.Add(comment); //here i found null exceptioin but it's not!
                user.Comments.Add(comment);
               
            }
            await _context.SaveChangesAsync();
            return Ok();
        }

in this line: qpost.Comments.Add(comment); i found the actual problem. it should work but i don’t understand why i found this kind of unexpected null exception. My comment parameter is contain:-

{
    "content": "This is a comment",
    "postId": 1
}

It should work dafinately, I don’t understand how to solve this annoying problem. I am an absolute beginner please help.

>Solution :

You need to include the comments when you retrieve QuestionPosts from database, otherwise the navigation property "Comments" will be null.

var qpost = await _context.QuestionPosts
    .Include(qp => qp.Comments)
    .FirstOrDefaultAsync(p => p.Id == comment.PostId);