The first time for MVVM and I got a problem. I need help

 { #if DEBUG             if (System.Diagnostics.Debugger.IsAttached)             {                 this.DebugSettings.EnableFrameRateCounter = true;             } #endif              Frame rootFrame = Window.Current.Content as Frame;              // Do not repeat app initialization when the Window already has content,             // just ensure that the window is active             if (rootFrame == null)             {                 // Create a Frame to act as the navigation context and navigate to the first page
                 rootFrame = new Frame();                 var viewModel = new MainWindowModel();                 MainPage main = new MainPage();                 main.DataContext = viewModel;
                 // TODO: change this value to a cache size that is appropriate for your application                 rootFrame.CacheSize = 1;                  if (e.PreviousExecutionState == ApplicationExecutionState.Terminated)                 {                     // TODO: Load state from previously suspended application                 }                  // Place the frame in the current Window                 Window.Current.Content = rootFrame;             }

 {         readonly List<Menu> _menu;          public MenuRespository()         {             if(_menu==null)             {                 _menu = new List<Menu>();             }             _menu.Add(Menu.CreateMenu("/Photos/Gas-50.png", "t"));             _menu.Add(Menu.CreateMenu("/Photos/Gas-50.png", "t"));             _menu.Add(Menu.CreateMenu("/Photos/Gas-50.png", "t"));             _menu.Add(Menu.CreateMenu("/Photos/Gas-50.png", "t"));             _menu.Add(Menu.CreateMenu("/Photos/Gas-50.png", "t"));             _menu.Add(Menu.CreateMenu("/Photos/Gas-50.png", "t"));             _menu.Add(Menu.CreateMenu("/Photos/Gas-50.png", "t"));         }         public List<Menu> GetMenu()         {             return new List<Menu>(_menu);         }     }

public class Menu     {         public static Menu CreateMenu(string iconthumb, string text)         {             return new Menu { IconThumb = iconthumb, Text = text };         }         public string IconThumb { get; set; }         public string Text { get; set; }     }

 public class MainWindowModel :ViewModelBase     {         readonly MenuRespository _menuRepository;         ObservableCollection<ViewModelBase> _viewModels;         public MainWindowModel()         {             _menuRepository = new MenuRespository();             //create an instance of our viewmodel add it to our collection             MenuListViewModel viewModel = new MenuListViewModel(_menuRepository);             this.ViewModels.Add(viewModel);         }         public ObservableCollection<ViewModelBase> ViewModels         {             get             {                 if(_viewModels==null)                 {                     _viewModels = new ObservableCollection<ViewModelBase>();                                      }                 return _viewModels;             }         }     }

 public abstract class ViewModelBase :INotifyPropertyChanged, IDisposable     {         protected ViewModelBase()         {          }         public event PropertyChangedEventHandler PropertyChanged;          protected virtual void OnPropertyChanged(string propertyName)         {             PropertyChangedEventHandler hander = this.PropertyChanged;             if(hander!=null)             {                 var e = new PropertyChangedEventArgs(propertyName);                 hander(this, e);             }         }          public void Dispose()         {             this.Dispose();         }          protected virtual void OnDispose()         {          }     }