DropDownList with Selected Me returns Null when doing post

I am working with ASP.Net MVC 5 and EF6, I have a class to edit users of the application and fill in a DropDownList the roles that exist and has preselected the role of the user, when doing post selecting another Role returns me a exception in the null view

  

Model.RolesList = 'Model.RolesList' started a type exception   'System.NullReferenceException'

This is the driver that brings the user's data to edit:

public ActionResult Edit(string Id)
    {
        //var Usuario = ctx.Users.Where(u => u.UserName.Equals(UserName, StringComparison.CurrentCultureIgnoreCase)).FirstOrDefault();
        var Usuario = UserManager.FindById(Id);

        var UsuarioRol = UserManager.GetRoles(Id).First();

        return View(new EditUsuarioViewModel() {
            Id = Usuario.Id,
            Email = Usuario.Email,
            Nombre = Usuario.Nombre,
            Apellido = Usuario.Apellido,
            AreaId = Usuario.AreaId,
            Alias = Usuario.Alias,
            //RolId = Usuario.RolId,
            //Para el llenado del comboBox de roles
            RolesList = ctx.Roles.ToList().Select(x => new SelectListItem()
            {
                Selected = UsuarioRol.Contains(x.Name),
                Text = x.Name,
                Value = x.Name
            })
        });
    }

RolesList returns the list of Roles and preselects the role of the user, until here everything works correctly, here's the post:

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit([Bind(Include= 
        "Id,Email,Alias,Nombre,Apellido,AreaId,RolId")]
        EditUsuarioViewModel editUser)
    {
       if(ModelState.IsValid)
        {
            var Usuario = UserManager.FindById(editUser.Id);

            if(Usuario == null)
            {
                return HttpNotFound();
            }

            Usuario.Email = editUser.Email;
            Usuario.UserName = editUser.Email;
            Usuario.AreaId = editUser.AreaId;
            Usuario.Nombre = editUser.Nombre;
            Usuario.Apellido = editUser.Apellido;
            Usuario.Alias = editUser.Alias;
            Usuario.RolId = editUser.RolId;

            //Verificamos si tiene un rol
            var UsuarioRol = UserManager.GetRoles(editUser.Id);

            //Si tiene rol pues lo quitamos
            if (UsuarioRol != null)
            {
                var quitar = UserManager.RemoveFromRoles(editUser.Id, UsuarioRol.ToArray());
                if(!quitar.Succeeded)
                {
                    ModelState.AddModelError("", quitar.Errors.First());
                    return View();
                }
            }

            //Agregamos el nuevo rol
            var result = UserManager.AddToRole(editUser.Id, editUser.RolId);
            if(!result.Succeeded)
            {
                ModelState.AddModelError("", result.Errors.First());
                return View();
            }

            return RedirectToAction("Index");
        }

        ModelState.AddModelError("", "No se pudo realizar los cambios");
        return View();
    }

the EditUsuarioViewModel class:

public class EditUsuarioViewModel
{
    public string Id { get; set; }

    [Required]
    [Display(Name = "Nombre del usuario")]
    public string Nombre { get; set; }

    [Required]
    [Display(Name = "Apellido del usuario")]
    public string Apellido { get; set; }

    [Required]
    [Display(Name = "Área")]
    public int AreaId { get; set; }

    [Required]
    [EmailAddress]
    [Display(Name = "Correo electrónico")]
    public string Email { get; set; }

    public string UserName { get; set; }

    [Required]
    [Display(Name = "Alias del Usuario")]
    public string Alias { get; set; }

    [Required]
    [Display(Name = "Rol")]
    public string RolId { get; set; }

    public string Foto { get; set; }

    public IEnumerable<SelectListItem> RolesList { get; set; }
}

In the view I create the DropDownList like this:

<div class="form-group">
            @Html.LabelFor(model => model.RolId, new { @class = "control-label col-md-2" })
            <div class=" col-md-10">
               <!-- AQUI ES DONDE MARCA EL ERROR -->
               @Html.DropDownListFor(model => model.RolId,Model.RolesList,"Seleccione un Rol")  
            </div>
        </div>

How can I be failing?

Greetings.