message: Close References window if main window is closed
  Fixes
diff: @@ -769,6 +769,10 @@ PoeditFrame::~PoeditFrame()
  {
  ms_instances.erase(this);
  + // don't leave file references window as the only one open:
  + if (ms_instances.empty() && FileViewer::GetIfExists())
  + FileViewer::GetIfExists()->Close();
  +
  DestroyContentView();
  wxConfigBase *cfg = wxConfig::Get();
  