Add list to object in Entity Framework

I have two Models called "Client" and "Software". Clients can have the same Software and Software can be on multiple clients. I want to add a List of Software to a specific Client.

Client Model:

public class Client
    {
        [Key]
        public int id { get; set; }
        [Required]
        public ICollection<Software>? Softwares { get; set; }
    }

Software Model:

public class Software
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int id { get; set; }
        public ICollection<Client>? Clients { get; set; }
    }

Why does this throw a NullReferenceException?

public void submit()
        {
            using (var repo = new ClientRepository(contextFactory.CreateDbContext()))
            {
                client = repo.getClientByID(Convert.ToInt32(clientid));

                foreach (var software in toAddList)
                {
                    client.Softwares.Add(software); //Error occurs here
                }

            }               
        }

Repo Code

public Client getClientByID(int id)
        {
            return _context.Clients.Find(id);
        }

>Solution :

Because you need to include Softwares when fetching from Clients table in database

In your repo, change your code to be like this :

public Client getClientByID(int id)
{
     return _context.Clients.Include(k => k.Softwares).First(i => i.id == id);
}