当调用事件操作时，不会触发WPF应用程序侦听方法

我有一个WPF MVVM应用程序。我有一个类叫MainWindowViewModel，还有一个类叫做TileMenuViewModel，它们都是从BaseViewModel派生而来的。基础所做的只是实现INotifyPropertyChanged和INotifyDataErrorInfo，同时保留一个错误的Dictionary<string, List<string>>进行验证。

在我的MainWindowViewModel中，我实例化了的一个实例

private TileMenuViewModel _tileMenuViewModel = new TileMenuViewModel();

在我的TileMenuViewModel中，我有一个public event Action NavToCampaings = new delegate {};

现在在MainWindowViewModel构造函数中，我订阅了以下事件：

_tileMenuViewModel.NavToCampaings += OnNavToCampaigns;

然而，当我在某个时刻从TileMenuViewModel调用事件时，就像NavToCampaigns();一样，我的订阅事件（MainWindowViewModel的OnNavToCampaigns）不会被调用。当我使用断点进行调试时，我可以看到NavToCampaigns();事件Action被调用，但其值为null。调试器在通过MainWindowViewModel构造时也不会进入TileMenuViewModel，我正在该构造中实例化TileMenuViewModel。然而，我的程序的其余部分按我的意愿工作，我看到平铺菜单也在工作。

我一辈子都搞不清楚自己做错了什么。有什么想法吗？为了简洁起见，这里是我的三个类：

MainWindowViewModel.cs

namespace MyApp.Main
{
    public class MainWindowViewModel : BaseViewModel
    {
        private string _windowsUser = WindowsIdentity.GetCurrent().Name;
        private TileMenuViewModel _tileMenuViewModel = new TileMenuViewModel();
        private CmpCampaignListViewModel _cmpCampaignListViewModel = new CmpCampaignListViewModel();
        private NapTransferCampaignListViewModel _ntCampaignListViewModel = new NapTransferCampaignListViewModel();
        public string WindowsUser { get { return _windowsUser; } }
        public string WindowTitle { get; set; }
        private BaseViewModel _currentViewModel;
        public BaseViewModel CurrentViewModel
        {
            get { return _currentViewModel; }
            set { SetProperty(ref _currentViewModel, value); }
        }
        public MainWindowViewModel()
        {
            WindowTitle = String.Format("MyApp - {0}", WindowsUser);
            NavCommand = new RelayCommand<string>(OnNav);
            AddNewCmpCampaignCommand = new RelayCommand(OnAddNewCmpCampaign);
            CurrentViewModel = _tileMenuViewModel;
            _tileMenuViewModel.NavToCampaings += OnNavToCampaigns;
        }
        public RelayCommand<string> NavCommand { get; private set; }
        public RelayCommand AddNewCmpCampaignCommand { get; private set; }
        public void OnNav(string destination)
        {
            switch (destination)
            {
                case "CmpCampaignList":
                    CurrentViewModel = _cmpCampaignListViewModel;
                    break;
                case "NtCampaignList":
                    CurrentViewModel = _ntCampaignListViewModel;
                    break;
                case "TileMenu":
                    CurrentViewModel = _tileMenuViewModel;
                    break;
                default:
                    CurrentViewModel = _tileMenuViewModel;
                    break;
            }
        }
        public void OnAddNewCmpCampaign()
        {
            var editCampaignWindow = new AddEditCmpCampaignWindow();
            editCampaignWindow.Show();
        }
        private void OnNavToCampaigns()
        {
            CurrentViewModel = _cmpCampaignListViewModel;
            //OnNav("CmpCampaignList");
        }
    }
}

TileMenuViewModel.cs

namespace MyApp.Main
{
    public class TileMenuViewModel : BaseViewModel
    {
        public TileMenuViewModel()
        {
            NavToCampaignsCommand = new RelayCommand(OnNavToCampaigns);
        }
        public RelayCommand NavToCampaignsCommand { get; private set; }
        public event Action NavToCampaings = delegate { };
        private void OnNavToCampaigns()
        {
            NavToCampaings();
        }
    }
}

BaseViewModel.cs

namespace MyApp.Infrastructure
{
    public class BaseViewModel : INotifyPropertyChanged, INotifyDataErrorInfo
    {
        private Dictionary<string, List<string>> _errors = new Dictionary<string, List<string>>(
            );
        public event PropertyChangedEventHandler PropertyChanged = delegate { };
        public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged = delegate { };
        public bool HasErrors
        {
            get { return _errors.Count > 0; }
        }
        public IEnumerable GetErrors(string propertyName)
        {
            return _errors.ContainsKey(propertyName) ? _errors[propertyName] : null;
        }
        protected virtual void SetProperty<T>(ref T member, T val,
            [CallerMemberName] string propertyName = null)
        {
            if (!object.Equals(member, val))
            {
                member = val;
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
            ValidateProperty(propertyName, val);
        }
        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        private void ValidateProperty<T>(string propertyName, T value)
        {
            var results = new List<ValidationResult>();
            ValidationContext context = new ValidationContext(this);
            context.MemberName = propertyName;
            Validator.TryValidateProperty(value, context, results);
            if (results.Any())
            {
                _errors[propertyName] = results.Select(c => c.ErrorMessage).ToList();
            }
            else
            {
                _errors.Remove(propertyName);
            }
            ErrorsChanged(this, new DataErrorsChangedEventArgs(propertyName));
        }
    }
}

DataContext = new TileMenuViewModel();