Kovarnii AfxMessageBox

I have multithreaded application. I get exeption when AfxMessageBox is called. The crash happens in func. CWinApp::DoMessageBox (it is called from AfxMessageBox)

// restore prompt context if possible
if (pdwContext != NULL)
*pdwContext = dwOldPromptContext; -> crash is here

When I use MessageBox - it works ok.
But I don't think that the problem is in AfxMessageBox, but in threads' syncronization.
Maybe you got some ideas?

It happens in debug (in release I have not checked yet). But I have something like:
AfxMessageBox("test");

No, there is no GetParent. The code goes like that:

HRESULT CMyClass::WaitForEnd()
{
MSG msg;

while (TRUE)
{
while (PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
{
TranslateMessage (&msg);
DispatchMessage (&msg);
}

//checks the flag signaled when the event is finished
if (((CMyApp *)AfxGetApp())->GetEndFlagState())
break;
}

return S_OK;
}


void CMyView::OnCancel()
{

if (AfxMessageBox ("Hello", MB_YESNO) == IDNO)
return;

if (!m_pDoc->m_ThreadEnd)
{ // the thread did not finish its work, so we can not yet
// return to the container
// tell the thread to finish its work
m_bIsCancel = TRUE;
m_pDoc->m_abortEvent->SetEvent();
}
else
{
m_bIsCancel = TRUE;
// notify CMyThread to exit.
SetEvent(m_pDoc->m_pWorkerThread->m_evnExit);
// signal flag for returning to container
((CMyApp *)AfxGetApp())->SetEndFlagState();
}
}

void CMyDoc::ActivateMyThread()
{
//create thread ...

// send to the worker thread its document parent
m_pWorkerThread->SetParentDoc(this);
m_pWorkerThread->DoWork();

while (TRUE)
{
while (PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
{
TranslateMessage (&msg);
DispatchMessage (&msg);
}

// checks the flag signaled when the work is finished
dwEventRet = WaitForSingleObject(m_pWorkerThread->m_evnNextFilter, 0);

if (dwEventRet == WAIT_OBJECT_0)
{
break;
}

if (((CMyApp *)AfxGetApp())->GetEndFlagState())
{
pdwFilterColorArr->RemoveAll();
pdwFilterColorArr=NULL;
return;
}
}
m_pWorkerThread->m_evnNextFilter.ResetEvent();
m_pWorkerThread->m_evnExit.SetEvent();
}


Notice - if I change AfxMessageBox by MessageBox - no crash occurrs. This may be the solution - but it is interesting - why this problem happens.