C# linq DefaultIfEmpty() NullReferenceException Value cannot be empty. Parameter name: Source [duplicate]

I have a problem How can I inquire in a linq correctly Get a mistake NullReferenceException every time

    public class Model
    {
        public int Nmb { get; set; }
        public string _UserId { get; set; }
    }
    public static List<Model> _models_List { get; set; }

        string user = "test name";

        int test = _models_List.Where(o => o._UserId == user).Select(o => o.Nmb).DefaultIfEmpty(0).First();

        if (test == 0)
        {         
            Model obj = new Model();
            obj._UserId = user;
            obj.Nmb = 1;
            _models_List.Add(obj);
        }

I tried to correct the code like this But I get the same mistake NullReferenceException Value cannot be empty. Parameter name: Source

    int test = _models_List.Where(o => o._UserId == user).Select(o => o.Nmb).FirstOrDefault();

Please Help