StackOverflow ActiveX Exception Handling Rough Draft

**Question**

Why is `pWnd = (CWnd*)pMap->LookupPermanent(m_hWnd);` occasionally returning NULL in the CWnd::DestroyWindow() function causing an ASSERT statement to fail while trying to destroy a modal dialog?

**Initial Information**

I have an ActiveX control that implements an on screen keyboard.
This keyboard accepts both touch/click events and keystroke events from an attached hardware keyboard.
Input to the control is validated using an algorithm defined by the module that created the keyboard.

Touching the buttons on screen generates input that follows

    OnButtonUP()->AddCharacter()->ValidateText()

Typing on the physical keyboard generates input that follows

    PreTranslateMessage()->AddCharacter()->ValidateText()

The ValidateText() function fires an event that is handled by the module that created the keyboard.

I have added a try-catch block around the event firing to handle any errors that occur while the event is being processed.

**Confusing Behavior**

What I have found is that if an error is caused by using the USB Keyboard, when the user attempts to close the Keyboard a debug assert statement in CWnd::DestroyWindow() fails.
If the same error is caused by using the touch screen interface, the assert statement succeeds.


**Source Code Snippets**

The code for firing the event is as follows:

    try
    {
        m_control->OnValidate(&text, &isValid);
    }
    catch (...)
    {
        LOG_CRITICAL_ERROR(/*Format(*/L"Keyboard::ValidateText: Error occured while validating"/*, badText.c_str()).c_str()*/);
    }

Where the OnValidate() function is defined as:

    void OnValidate(BSTR* text, LONG* isValid)
    {
        FireEvent(eventidOnValidate, EVENT_PARAM(VTS_PBSTR VTS_PI4), text, isValid);
    }

The CWnd::DestroyWindow() function is (With some bits preprocessor bits removed) :

    BOOL CWnd::DestroyWindow()
    {
    	CWnd* pWnd;
    	CHandleMap* pMap;
    	HWND hWndOrig;
    	BOOL bResult;

    	if ((m_hWnd == NULL) && (m_pCtrlSite == NULL))
    		return FALSE;

    	bResult = FALSE;
    	pMap = NULL;
    	pWnd = NULL;
    	hWndOrig = NULL;
    	if (m_hWnd != NULL)
    	{
    		pMap = afxMapHWND();
    		ENSURE(pMap != NULL);
    		pWnd = (CWnd*)pMap->LookupPermanent(m_hWnd); //<- This returns NULL before the Assert fails
    		hWndOrig = m_hWnd;
    	}

    	if ((m_hWnd != NULL) || (m_pCtrlSite != NULL))
    	{
    		if (m_pCtrlSite == NULL)
    			bResult = ::DestroyWindow(m_hWnd);
    		else
    			bResult = m_pCtrlSite->DestroyControl();
    	}

    	if (hWndOrig != NULL)
    	{
    		// Note that 'this' may have been deleted at this point,
    		//  (but only if pWnd != NULL)
    		if (pWnd != NULL)
    		{
    			// Should have been detached by OnNcDestroy
    			ASSERT(pMap->LookupPermanent(hWndOrig) == NULL);
    		}
    		else
    		{
    			ASSERT(m_hWnd == hWndOrig); //<-This is the ASSERT that fails!
    		// Detach after DestroyWindow called just in case
    			Detach();
    		}
    	}

    	return bResult;
    }

From what I have been able to obtain via tracepoints, when the assert fails, `pWnd = (CWnd*)pMap->LookupPermanent(m_hWnd);` returns NULL.