And we finally edit the lifecycle handlers of the extensions to suit our needs

CustomFormPanelCommandSet_PART3.ts

// imports ...
export default class CustomFormPanelCommandSet extends BaseListViewCommandSet<ICustomFormPanelCommandSetProperties> {
//...
  @override
  public onListViewUpdated(event: IListViewCommandSetListViewUpdatedParameters): void {
    const openEditorCommand: Command = this.tryGetCommand('CMD_PANEL');
    openEditorCommand.visible = event.selectedRows.length === 1;
  }

  @override
  public onExecute(event: IListViewCommandSetExecuteEventParameters): void {

    switch (event.itemId) {
      case 'CMD_PANEL':
        let selectedItem = event.selectedRows[0];
        const listItemId = selectedItem.getValueByName('ID') as number;
        const title = selectedItem.getValueByName("Title");
        this._showPanel(listItemId, title);
        break;
      default:
        throw new Error('Unknown command');
    }
  }
//...
}