Currently WebAqua will cause an error if we try to modify button collection during click event handler. A bug report has been submitted to the developer. We will inform you if there is any update or progress regarding this issue.

It doesn't seem that you've grasped the full scope of the issue.  The button collection seems to be unmodifiable from any event or code after it has been loaded.  NOT JUST A CLICK EVENT.  For example, the following code demonstrates adding a StackButton on a 5 second delay.  This will also crash the FishEye when the button is clicked.

        public MainPage()
        {
            InitializeComponent();

            AddStackButton();
            AddStackButtonWithDelay(5);
        }


        private void AddStackButtonWithDelay(double delayDuration)
        {
            Storyboard toolbarDelay = new Storyboard() { BeginTime = TimeSpan.FromSeconds(delayDuration) };
            toolbarDelay.Completed += (s, e) => AddStackButton();
            toolbarDelay.Begin();
        }


        
        private void AddStackButton()
        {
            WebFishEyeButton button = _MainToolbar.FindButton("cmdTools");

            if (button.StackButtons == null)
            {
                button.StackButtons = new ObservableCollection<WebFishEyeStackButton>();
            }

            string stackText = string.Format("StackButton{0}", _StackCounter++);

            WebFishEyeStackButton stackButton = new WebFishEyeStackButton();
            stackButton.ItemName = stackText;
            stackButton.Text = stackText;
            stackButton.ImageSource = new BitmapImage(new Uri("Resources/toolbox.128.png", UriKind.Relative));

            button.StackButtons.Add(stackButton);
        }

In your scenario, you will need to invoke the RefreshUIElement function in the WebFishEye in order to render the new stack button. Here is the modified snippet for the AddStackButtonWithDelay

private void AddStackButtonWithDelay(double delayDuration)
{
    Storyboard toolbarDelay = new Storyboard() { BeginTime = TimeSpan.FromSeconds(delayDuration) };
    toolbarDelay.Completed += (s, e) => { AddStackButton(); _MainToolbar.RefreshUIElement(); };
    toolbarDelay.Begin();
}

During OnClick event handler, you will also need to invoke this method as well. However, in the current version of WebFishEye this will result in an error. The proposed fix is to set the event parameter Handle property to True, currently the Handle property is not available. This fix will be available in the next build of WebFishEye.