Object not set to reference while unit testing API controller (asp.net 5, xunit)

Hi everyone I am new to testing so please go easy :). I'm having trouble testing my api controllers. I have created a separate class to test the controller but I having issues with 'Object reference not set to valid instance'

The api controller and method I want to test:

namespace Project.Controllers.Api
{
    [Authorize]
    [Route("/api/entries/{date}")]
    public class EntryController : Controller
    {
        private ILogger<EntryController> _logger;
        private ITrackerRepository _repository;

        public EntryController(ITrackerRepository repository, ILogger<EntryController> logger)
        {
            _repository = repository;
            _logger = logger;
        }

        [HttpGet("")]
        public JsonResult Get(string date)
        {
            // DateTime date mm/dd/yyyy
            DateTime dateTime = Convert.ToDateTime(date);
            var result = Mapper.Map<IEnumerable<EntryViewModel>>(_repository.GetDiaryEntries(dateTime, User.Identity.Name));

            if (result != null)
            {
                return Json(result);
            }
            else
            {
                return Json(null);
            }
        }
}

my attempt to unit test with xunit:

namespace test.Project.UnitTests { public class EntryControllerTest {

    public EntryControllerTest()
    {
    }

    [Fact]
    public void TestInvalidViewModels()
    {
        // Created test mock/empty repository
        TestProjectRepository testRepo = new TestProjectRepository();
        var controller = new EntryController(testRepo, null);
        var result = controller.Get("01/12/1899");
        Assert.Equal(result, null);
    }
}

}

I get the issue that 'Object reference is not set to an instance of an object" in the Controller.cs file. Any help much appreciated, thank you.