protected override void OnClick() {
     if (_isOpen)
         return;
     _overview = new OverviewWindow();
     _overview.ViewContent = MapControlContentFactory.Create(
         MapView.Active.Map, MapView.Active.Extent, MapView.Active.ViewingMode);
     _overview.Closed += (s, e) => {
         _isOpen = false;
         lock (_lock) {
             _overview = null;
         }
     };
     _overview.Show();
     _isOpen = true;
 }