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"]);
}