public PanelViewModelBase Open(PanelViewModelBase vm)
        {
            var panelViewModelBase = _panels.FirstOrDefault(p => p == vm);

            if (panelViewModelBase == null)
            {
                panelViewModelBase = vm;
                _panels.Add(panelViewModelBase);
            }

            ActiveDocument = panelViewModelBase;
            return(panelViewModelBase);
        }
 internal void Close(PanelViewModelBase panel)
 {
     _panels.Remove(panel);
 }