if (mPresShell)
if (mPresShell) {
DocumentViewerImpl::~DocumentViewerImpl(){  NS_ASSERTION(!mDocument, "User did not call nsIContentViewer::Close");  if (mDocument)    Close();  NS_ASSERTION(!mPresShell, "User did not call nsIContentViewer::Destroy");  if (mPresShell)    Destroy();  // XXX(?) Revoke pending invalidate events  // clear weak references before we go away  if (mPresContext) {    mPresContext->SetContainer(nsnull);    mPresContext->SetLinkHandler(nsnull);  }}