Oracle Forms

I have a super simple Web form. It basically only houses a custom menu used to call other forms. However, when I attempt to close this form by clicking the browser's X button I get a FRM-41039 Invalid Alert ID error.

I don't understand why? The only Alerts in the form were put there when I subclassed the WebUtil Object Class.

I even tried capturing the error in a on-error and on-message trigger just to try to suppress it. But it pops up no matter what I do.

Any known issues or troubleshooting advice would be greatly appreciated.