Return JSON string from IHttpActionResult in ASP.Net Web API

hello i am following this article

https://www.tutorialsteacher.com/webapi/implement-get-method-in-web-api

GetAllStudents () i want to change the return query like

        public IHttpActionResult GetAllUser()
        {
            //IList<UserViewModel> students = null;

            List<object> customers = new List<object>();

            using (SqlConnection conn = new SqlConnection())
            {
                conn.ConnectionString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
                using (SqlCommand cmd = new SqlCommand())
                {
                    cmd.CommandText = "SELECT * FROM users ";

                    cmd.Connection = conn;
                    conn.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        while (sdr.Read())
                        {
                            customers.Add(new
                            {
                                Id = sdr["id"],
                                username = sdr["username"],
                                password = sdr["password"]
                            });
                        }
                    }
                    conn.Close();
                }
               

                if (customers.Count == 0)
                {
                    return NotFound();
                }
                return (new JavaScriptSerializer().Serialize(customers));
                //return Ok(customers);
            }
        }

but its not returning the list and giving error as 

{"Message":"An error has occurred.","ExceptionMessage":"Object reference not set to an instance of an object.","ExceptionType":"System.NullReferenceException","StackTrace":"   at WebAPI.Controllers.UserController.GetAllUser() in C:\\Users\\Nausheen\\source\\repos\\WebAPI\\WebAPI\\Controllers\\UserController.cs:line 33\r\n   at lambda_method(Closure , Object , Object[] )\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass6_1.<GetExecutor>b__3(Object instance, Object[] methodParameters)\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Object instance, Object[] arguments)\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__5.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__15.MoveNext()"}

please guide how to return list in web api using the above method