NS_PRECONDITION(mWindow, "null window");
DocumentViewerImpl::Show(void){  NS_ENSURE_TRUE(mDocument, NS_ERROR_NOT_AVAILABLE);  NS_PRECONDITION(mWindow, "null window");  // We don't need the previous viewer anymore since we're not  // displaying it.  if (mPreviousViewer) {    // This little dance *may* only be to keep    // PresShell::EndObservingDocument happy, but I'm not sure.    nsCOMPtr<nsIContentViewer> prevViewer(mPreviousViewer);    mPreviousViewer = nsnull;    prevViewer->Destroy();  }  if (mWindow) {    mWindow->Show(PR_TRUE);  }  return NS_OK;}