NullReference durning testing controller with NUnit and Moq

I obtain a NullReference exception in my test. When I comment eventsRepository.AddEvent(eve, User.Identity.GetUserId()); in controller than it goes through.

How Can I fix it ?

Controller's Method

    [ValidateAntiForgeryToken]
    [HttpPost]
    [Authorize]
    public ActionResult CreateEvent(Event eve)
    {
        if (eve.DateOfBegining < DateTime.Now)
        {
            ModelState.AddModelError("DateOfBegining", "");
        }

        if (eve.MaxQuantityOfPlayers < eve.MinCount)
        {
            ModelState.AddModelError("MinCount", "");
        }

        if (eve.ConflictSides.Count < 2 || eve.ConflictSides.Count > 10)
        {
            ModelState.AddModelError("ConflictSides", "");
        }

        if (!ModelState.IsValid)
        {
            return View("CreateEvent", eve);
        }
        else
        {
            eventsRepository.AddEvent(eve, User.Identity.GetUserId());
            return RedirectToAction("EventsList");
        }
    }

AddEvent

void AddEvent(Event ev, string userId);

Test's method

  [TestMethod]
    public void CreateEvent_AddEvent_returns_EventsList()
    {
        // arrange
        var EventRepo = new Mock<IEventRepository>();
        var ParticipantsRepo = new Mock<IParticipants>();

        DateTime dt = new DateTime(2200, 1, 23);
        Event eve = new Event() 
        {
            ConflictSides = new List<ConflictSide>() { 
                                                    new ConflictSide{ Name ="niebiescy"},
                                                    new ConflictSide{ Name ="czerwoni"},
                                                    new ConflictSide{ Name ="fioletowi"},
                                                    },
        DateOfBegining = dt,
        Description = "bardzo dlugi opid na potrzeby testu",
        EventCreator= "userId",
        EventName = "najlepsza",
        FpsLimitInBuildings=300,
        FpsLimitOnOpenField=500,
        Hicap = new MagazineTyp(){ifAllow = true, ifOnlySemi = false},
        MidCap = new MagazineTyp(){ifAllow = true, ifOnlySemi = false},
        LowCap = new MagazineTyp(){ifAllow = true, ifOnlySemi = false},
        RealCap = new MagazineTyp(){ifAllow = true, ifOnlySemi = false},
        MaxQuantityOfPlayers = 50,
        MinCount = 10           
        };

        var target = new EventController(EventRepo.Object, ParticipantsRepo.Object);

        // act

        RedirectToRouteResult result = target.CreateEvent(eve) as RedirectToRouteResult;

        // assert

        // EventRepo.Verify(a => a.AddEvent(It.IsAny<Event>(), It.IsAny<string>()), Times.Once());

        Assert.AreEqual("EventsList", result.RouteValues["action"]);
    }