无法将文本块添加到堆栈面板子项

我有上面的代码，它以编程方式创建一个带有文本块、进度条和按钮的弹出窗口，然后显示。进度条和按钮还可以，但我添加到stackpanel.kids的文本块没有显示。有人对这个问题有想法吗？非常感谢。

                var tbInfo = new TextBlock {Text = "Checking... ", TextWrapping = TextWrapping.Wrap, Visibility = Visibility.Visible};
            var mainPanel = new StackPanel {Background = new SolidColorBrush(Colors.Black)};
            var popupBtnCancle = new Button {Content = "Cancel", Margin = new Thickness(3)};
            popupBtnCancle.Click += new RoutedEventHandler(popupBtnCancle_Click);
            var pbLoading = new ProgressBar
            {
                IsIndeterminate = true,
                IsEnabled = true,
                Visibility = Visibility.Visible,
            };

            mainPanel.Children.Add(tbInfo);
            mainPanel.Children.Add(pbLoading);
            mainPanel.Children.Add(popupBtnCancle);
            border.Child = mainPanel;
            _loading.Child = border;
            _loading.VerticalOffset = 0;
            _loading.HorizontalOffset = 0;
            _loading.IsOpen = true;