Crash in CFileDailog when called from hook procedure (OFNHookProc)

I have created a customzied CfileDialog dialog box by attaching a
child dialog to it using the hook procedure (OFNHookProc (....) ). Now
that child dialog also has a browse button that also uses CfileDialog
to select another file. This is done in repsonse to the WM_COMMAND
message. The creation of the modal dialog box is fine. It does all
the operations for selecting the file but when I select the concerned
file and press the open button then the application crashes and the
control goes to objcore.cpp as shown below

BOOL CObject::IsKindOf(const CRuntimeClass* pClass) const
{
    ASSERT(this != NULL);
    // it better be in valid memory, at least for CObject size
    ASSERT(AfxIsValidAddress(this, sizeof(CObject)));

    // simple SI case
    CRuntimeClass* pClassThis = GetRuntimeClass();
    return pClassThis->IsDerivedFrom(pClass);
}

It is not able to ASSERT(this != NULL); statement and hence the
application crashes.

I am not able to figure out the exact cause of the error.

Thanks,
Shyam