对象引用未设置为具有类方法的对象的实例

Question

我正在尝试将类对象绑定到WPF应用程序。但它每次都会抛出方法中的异常。在下面的代码中发生异常的注释。有什么建议吗？

namespace Explorer
{
    class DirectoryTree : INotifyPropertyChanged
    {
        //members
        private string _path;
        private string _name;
        private ObservableCollection<DirectoryTree> _SubDirectories;
        //private ObservableCollection<FileInfo> _files;


        //properties
        public string Path
        {
            get { return _path; }
            set
            {
                if(_path != value)
                {
                    _path = value;
                    if (PropertyChanged != null)
                        PropertyChanged(this, new PropertyChangedEventArgs("Path"));
                }
            }
        }


        public string Name
        {
            get { return _name; }
            set
            {
                if (_name != value)
                {
                    _name = value;
                    if (PropertyChanged != null)
                        PropertyChanged(this, new PropertyChangedEventArgs("Name"));
                }
            }
        }


        public ObservableCollection<DirectoryTree> SubDirectories
        {
            get { return _SubDirectories; }
            set
            {
                if (_SubDirectories != value)
                {
                    _SubDirectories = value;
                    if (PropertyChanged != null)
                        PropertyChanged(this, new PropertyChangedEventArgs("SubDirectories"));
                }
            }
        }


        //method to process the directory recursively
        public DirectoryTree TraverseDirectory(DirectoryInfo rootdirectory)
        {
            DirectoryTree directoryTree = new DirectoryTree();
            directoryTree.Path = rootdirectory.FullName;
            directoryTree.Name = directoryTree.Path.Substring(directoryTree.Path.LastIndexOf("\\") + 1);

            if(directoryTree !=null)
            {
                //process all the files under this directory
                List<FileInfo> files = new List<FileInfo>(rootdirectory.GetFiles("*.*").ToList());
                if (files.Count > 0)
                {
                    foreach (FileInfo f in files)
                    {
                        try
                        {
                            DirectoryTree fileInTree = new DirectoryTree();
                            fileInTree.Path = f.Directory.ToString();
                            fileInTree.Name = f.Name;
                            directoryTree.SubDirectories.Add(fileInTree);//object reference not set to an instance of an object
                        }
                        catch (Exception ex)
                        {
                            MessageBox.Show(ex.Message);
                        }
                    }
                }


                //process all the directories under the current directory
                List<DirectoryInfo> directories = new List<DirectoryInfo>(rootdirectory.GetDirectories().ToList());
                if (directories.Count > 0)
                {
                    foreach (DirectoryInfo d in directories)
                    {
                        DirectoryTree directoryInTree = new DirectoryTree();
                        directoryInTree.Path = d.FullName;
                        directoryInTree.Name = d.Name;
                        directoryTree.SubDirectories.Add(directoryInTree);
                        TraverseDirectory(d);
                    }
                }
            }

            return directoryTree;
        }

        public event PropertyChangedEventHandler PropertyChanged;


    }
}