xUnit test error System.NullReferenceException when creating a viewModel instance to test a controller

Question

I want to test the Create(ClienteViewModel cliente), and I'm having difficulty creating an instance of it. I have tried to mock the viewModel, but returns me a different error.

The code of the test I'm getting the error from:

        [Fact]
        public void Post_Create_Client()
        {
            var controller = new PoollGest.Controllers.ClientesController(_context);
            // Arrange
          /*  var mockRepo = new Mock<ClienteViewModel>();
            mockRepo.SetupProperty(m => m.Nome, "nome");
            mockRepo.SetupProperty(m => m.Desconto, 0);*/
          ClienteViewModel viewModel = new ClienteViewModel { 
                  Nome ="nome",
                  Desconto = 0,
          };

            // Act
            var result =  controller.Create(viewModel);

            // Assert
            var viewResult = Assert.IsType<ViewResult>(result);
            var model = Assert.IsType<ClienteViewModel>(viewResult.ViewData.Model);
            Assert.Equal("nome", model.Nome);
            Assert.Equal(0, model.Desconto);
        }
    }

Controller, the Create Action I'm testing:

 [HttpPost]
        [ValidateAntiForgeryToken]
        public IActionResult Create(ClienteViewModel cliente)
        {
            if (ModelState.IsValid)
            {
                Cliente c = new Cliente
                {
                    Nome = cliente.Nome,
                    Desconto = cliente.Desconto,
                    DataNascimento = cliente.DataNascimento,
                    NIF = cliente.NIF
                };
                _context.Add(c);
                _context.SaveChanges();
                TempData["success"] = true;
                TempData["msg"] = "Cliente adicionado com sucesso.";
                return RedirectToAction(nameof(Index));
            }
            TempData["error"] = true;
            TempData["msg"] = "Ocorreu um problema.";
            return View(cliente);
        }

Error's I'm getting:

NullReferenceException:

If I mock the viewModel, change the call to Create(mockRepo.Object), gives me another error

Before doing this test the VerifyModelState worked