Insert Data Error EntityFramework Core

Sorry if the question is very obvious but I'm new to the subject and looking in Google I can not find anything about what happens.

I have the Next entity.

public class Equipo
    {
        public int Id { get; set; }
        public string Serial { get; set; }
        public string Descripcion { get; set; }
        public int IdTipoEquipo { get; set; }
        public int IdMarca { get; set; }
        public int IdRango { get; set; }
        public int IdResponsable { get; set; }
        public decimal PeriodoCalibracion { get; set; }
        public decimal Clase { get; set; }
        public string Patron { get; set; }
        public string ActivoFijo { get; set; }
        public string IdArticulo { get; set; }
        public int IdEstado { get; set; }
        public string UsuarioRegistro { get; set; }
        public DateTime FechaRegistro { get; set; } = DateTime.Now;
        public string UsuarioActualiza { get; set; }
        public DateTime FechaActualiza { get; set; } = DateTime.Now;
        public List<EquipoDetalle> Detalle { get; set; }
    }

As you see inside that entity goes another call detail that is this.

public class EquipoDetalle
    {
        public int Id { get; set; }
        [Column("IdEquipo")]
        public int EquipoId { get; set; }
        public DateTime Fecha { get; set; }
        public int IdLaboratorio { get; set; }
        public string Certificado { get; set; }
        public string Observacion { get; set; }
        public DateTime? ProxVerificacion { get; set; }
        public int IdMovimiento { get; set; }
        // public byte[] Imagen { get; set; }
        public int? IdCalibracion { get; set; }
        public int IdResponsable { get; set; }
        public int IdEstado { get; set; }
        public string UsuarioRegistro { get; set; }
        public DateTime FechaRegistro { get; set; } = DateTime.Now;
        public string UsuarioActualiza { get; set; }
        public DateTime FechaActualiza { get; set; } = DateTime.Now;
        public Programacion Programacion { get; set; }
    }

And the detail has another program call that is this.

public class Programacion
    {
        public int Id { get; set; }
        public string IdEmpleado { get; set; }
        public DateTime Fecha { get; set; }
        public int IdEquipoDetalle { get; set; }
        public int IdEstado { get; set; }
        public string UsuarioRegistro { get; set; }
        public DateTime FechaRegistro { get; set; }
        public string UsuarioActualiza { get; set; }
        public DateTime FechaActualiza { get; set; }
    }

That's where I have the problem, since the insert I want to make is only of Equipment and detail but not programming, programming goes null but at the moment I do the savechanges () it gives me an error and it's because programming go null, I do not know how to tell the entity to just insert the entities I want and not the last one.

I appreciate your guide since I am very new with entityFramework and I do not know how to look.

This is the insert method.

public int Add(Equipo item)
        {
            // Detalle
            item.Detalle = new List<EquipoDetalle>{
                    new EquipoDetalle{
                        Fecha= item.FechaRegistro,
                        IdLaboratorio = 1,
                        Observacion= "Registro de Equipo",
                        IdMovimiento = 1,
                        IdResponsable = item.IdResponsable,
                        IdEstado = item.IdEstado,
                        UsuarioRegistro= item.UsuarioRegistro,
                        UsuarioActualiza=item.UsuarioActualiza,
                        FechaRegistro=item.FechaRegistro,
                        FechaActualiza = item.FechaActualiza
                }
                    };
            try
            {
                _context.Equipo.Add(item);
                return _context.SaveChanges();
            }
            catch (Exception ex)
            {
                return 1;
            }

        }

As you can see, the item is full and the detail urges, if I comment on the programming property, it does not give me an error since it does not try to do the insert.

This is the relationship.