private void OnImportEXCEL(object _type)
        {
            if (_type == null)
            {
                return;
            }
            NodesToExcelExportType type = NodeAssembler.StringToType(_type.ToString());

            try
            {
                var dlg = new Microsoft.Win32.OpenFileDialog()
                {
                    Filter = "excel files|*.xls;*.xlsx"
                };
                // this causes a Securiy Exception (SQLite) if the propgram is not run as Administrator,
                // but it has no effect on the rest
                if (dlg.ShowDialog().Value)
                {
                    if (File.Exists(dlg.FileName))
                    {
                        // get the name of the table
                        string tablename = "Tabelle1";
                        Window window    = Application.Current.MainWindow;
                        if (window != null)
                        {
                            MainWindow mw = window as MainWindow;
                            if (mw != null)
                            {
                                tablename = mw.OpenTableNameWindow();
                            }
                        }

                        //imports the EXCEL file
                        ExcelImporter         excelImp = new ExcelImporter();
                        List <List <string> > raw_data = excelImp.ImportFromFile(dlg.FileName, tablename, 1000);

                        // extract the data
                        switch (type)
                        {
                        case NodesToExcelExportType.COMPLETE:
                            this.imported_node_list = NodeAssembler.GetNodeListComplete(raw_data);
                            break;

                        case NodesToExcelExportType.EXPLICIT:
                            this.imported_node_list = NodeAssembler.GetNodeListExplicit(raw_data);
                            break;

                        default:
                            this.imported_node_list = NodeAssembler.GetNodeList(raw_data);
                            break;
                        }

                        // attach to the nodemanager
                        NodeAssembler.AttachNodesToNodeManager(this.imported_node_list, ref this.NManager);

                        // publish to GUI
                        this.OnRefreshTree();
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "EXCEL File Import Error", MessageBoxButton.OK, MessageBoxImage.Error);
                // MessageBox.Show(ex.StackTrace, "EXCEL File Import Error", MessageBoxButton.OK, MessageBoxImage.Error);
            }
        }