public async Task RegisterActivityReturnsCorrectJsonWhenModelStateIsNotValid()
        {
            const string modelStateErrorMessage = "modelStateErrorMessage";

            var sut = new ActivityApiController(null);
            sut.AddModelStateError(modelStateErrorMessage);

            var jsonResult = (JsonResult)await sut.RegisterActivity(new ActivitySignupViewModel());
            var result = jsonResult.GetValueForProperty<List<string>>("errors");

            Assert.IsType<JsonResult>(jsonResult);
            Assert.IsType<List<string>>(result);
            Assert.Equal(result.First(), modelStateErrorMessage);
        }