public IActionResult AddExistingExternalUserToStep(AddResponsibleUserToStepDTO addExternalUserToStep)
        {
            Response.StatusCode = StatusCodes.Status202Accepted;

            if (string.IsNullOrEmpty(addExternalUserToStep.Step))
            {
                Response.StatusCode = StatusCodes.Status400BadRequest;

                return(PartialView("~/Views/User/Partials/_AddNewUser.cshtml"));
            }

            var existingUser = addExternalUserToStep.ExistingResponsibleUser;

            if (TryValidateModel(existingUser))
            {
                var result = _planRepository.AddUserToPlanStep(existingUser.Id.Value, addExternalUserToStep.PlanId, addExternalUserToStep.Step);

                if (result)
                {
                    Response.StatusCode = StatusCodes.Status201Created;
                }
                else
                {
                    ModelState.AddModelError(string.Empty, sharedResource.userAddExistingExternalUserToStepProblem);
                }
            }

            return(PartialView("~/Views/User/Partials/_AddExistingUser.cshtml"));
        }