EF Code first: Insert Many to many
===

A post can have many Topics. A topic can be assigned to many posts.
When adding a post with two topics selected from topic list, two NULL topics also inserted to my topic table. See Id=34 and 35. What did I do wrong?
 

Id	TopicName	TopicDesc		
31	Sports	Sports	
32	Game	Game	
33	Politics	Politics	
34	NULL	NULL	
35	NULL	NULL	

    public class Post
    {
        public int Id { get; set; }
        public int UserId { get; set; }
       
        public virtual ICollection<Topic> PostTopics { get; set; }

    }


    public class Topic
    {
        public int Id { get; set; }
        public string TopicName { get; set; }

        public virtual ICollection<Request> Requests { get; set; }

    }

    // insert code: I think the problem is here

      using (var context = new ChatContext())
      {
                 // Post
                 context.Posts.Add(pobjPost);

                 pobjPost.PostTopics = new List<Topic>();
                 // topics
                 foreach (var i in pobjTopics)
                 {
                
                 pobjPost.PostTopics.Add(i);
                 }
                
                 context.SaveChanges();
       }