Update MainVM.cs

Author: yanjinhuagood

src/WPFDevelopers.Samples.Shared/ViewModels/MainVM.cs

@@ -315,6 +315,9 @@ void MenuItemSelection(string _menuName)
                 case MenuEnum.TimePicker:
                     ControlPanel = new TimePickerExample();
                     break;
+                case MenuEnum.WaterfallPanel:
+                    ControlPanel = new WaterfallPanelExample();
+                    break;
                 case MenuEnum.VirtualizingWrapPanel:
                     ControlPanel = new VirtualizingWrapPanel();
                     new VirtualizingWrapPanelExample().MaskShowDialog();