System.Runtime.InteropService.COMException thrown and Internet Explorer closes

Question

I'm using SHDocVw.InternetExplorer to open and control a browser window for automation on an internal website. The software generally performs as desired but the browser will close occasionally. I have tracked down the lines of code that cause it to happen but I'm unable to think of a way to fix the problem.

My original code looked like this:

private InternetExplorer window;

public HTMLDocument Document 
{ 
    get 
    {
        try
        {
            return (HTMLDocument)window.Document;
        }
        catch (System.Runtime.InteropServices.COMException)
        {
            return null;
        }
    } 
}

However, whenever the the window.Document call resulted in a COMException the browser would close. I tried creating a isReady boolean that was set to false when I tried to navigate away from the current page and was set true when the NavigateComplete2 event was fired but it doesn't fire if the browser asks "are you sure you want to leave this page" which meant that I couldn't interact with those popups.

So I tried changing the code to:

private InternetExplorer window;

public HTMLDocument Document 
{ 
    get 
    {
        try
        {
            if (window.Busy) return null;
            return (HTMLDocument)window.Document;
        }
        catch (System.Runtime.InteropServices.COMException)
        {
            return null;
        }
    } 
}

While I've experienced fewer crashes it will still occasionally close the browser when a COMException occurs on the window.Busy call.

Since NavigateComplete2 doesn't fire when the navigation is interrupted by the browser and InternetExplorer.Busy can also result in the browser closing I'm out of ideas on how to resolve.

Edit: I have now also tried: window.ReadyState == tagREADYSTATE.READYSTATE_COMPLETE but it throws the same COMException on window.ReadyState and crashes the windows.