Assert that a Twitter bootstrap dialog is visible after clicking a button

Dialog.cs

    public class Dialog<TModel> : UiComponent<TModel>
        where TModel : class, new()
    {

        private IWebElement Element
        {
            get
            {
                return this.Find.Element(By.CssSelector(".dialog.modal"));
            }
        }

        public bool Visible
        {
            get
            {
                // always returns false.
                // TODO: Thread.Sleep(500);
                return this.Element.Displayed;
            }
        }
    }

GroupTests.cs

    [TestClass]
    public partial class GroupTests
    {
        protected IndexPage<GroupModel> indexPage;
        protected SampleData<GroupModel> sampleData;
        protected Row<GroupModel> row;

        [TestInitialize]
        public void Initialize()
        {
            // navigate to the index page
            indexPage = Host.Instance
                        .NavigateToInitialPage<GroupsController, IndexPage<GroupModel>>(
                            c => c.Index(),
                            new { area = "Admin" }
                        );

            // create a model with sample data
            sampleData = new SampleData<GroupModel>();

            // select a random row
            row = indexPage.Table.RandomRow();
        }

        [TestClass]
        public partial class AddButton : GroupTests
        {
            [TestMethod]
            public void ShouldBeDisplayed()
            {
                indexPage.AddButton.Displayed.Should().BeTrue(
                    reason: "Add button should be visible"
                );
            }

            [TestMethod]
            public void ShouldDisplayDialog_WhenClicked()
            {
                indexPage.AddButton.Click();

                // TODO: Wait about 500 milliseconds before asserting
                indexPage.Dialog.Visible.Should().BeTrue(
                    reason: "The modal dialog should be displayed after clicking the 'Add' button"
                );

                indexPage.Dialog.Cancel();
            }
        }
    }