空引用异常c＃wp7

private void button2_Click(object sender, RoutedEventArgs e)
    {
        ViewDiskModel model =  this.LayoutRoot.DataContext as ViewDiskModel;
        int m_iSelectedLoad = lstBox1.SelectedIndex;
        if (m_iSelectedLoad >= 0)
        {
            model.DeleteSelectedFiles.Execute(null);

        }

        MessageBox.Show(\"Files Successfully Deleted\");
    }

namespace WindowsPhoneApplication1.Model
{
    public class AddFileModel : ModelBase
    {
        private string _filename;
        public string FileName
        {
            get
            {
                return this._filename;
            }
            set
            {
                this._filename = value;
                this.OnPropertyChanged(\"FileName\");
            }

        }

        private string _filetext10;
        public string FileText10
        {
            get
            {
                return this._filetext10;
            }
            set
            {
                this._filetext10 = value;
                this.OnPropertyChanged(\"FileText10\");
            }
        }

        private ICommand _saveFile;
        public ICommand SaveFile
        {
            get
            {
                this._saveFile = this._saveFile ?? new DelegateCommand(this.OnSaveFile);
                return this._saveFile;
            }
        }

         private ICommand _readSelectedFiles;
        public ICommand ReadSelectedFiles
        {
            get
            {
                this._readSelectedFiles = this._readSelectedFiles ?? new DelegateCommand(this.OnReadSelected);
                return this._readSelectedFiles;
            }
        }

        private void OnSaveFile()
        {
            if (!string.IsNullOrEmpty(this.FileName))
            {
                using (var store = IsolatedStorageFile.GetUserStoreForApplication())
                {
                    if (store.FileExists(FileName))
                        store.DeleteFile(FileName);

                    using(StreamWriter writer = new StreamWriter(new IsolatedStorageFileStream(FileName, FileMode.OpenOrCreate, store)))
                    {
                        writer.WriteLine(this.FileText1 + this.FileText2 + this.FileText3 + this.FileText4 + this.FileText5 + this.FileText6 + this.FileText7 + this.FileText8 + this.FileText9 + this.FileText10);
                        writer.Close();
                    }
                }
            }
        }

        private void OnReadSelected()
        {
            using (var store = IsolatedStorageFile.GetUserStoreForApplication())
            {
                if (store.FileExists(FileName))
                {
                    using (StreamReader reader = new StreamReader(new IsolatedStorageFileStream(FileName, FileMode.OpenOrCreate, store)))

                        {
                            string textData = this.FileText1 + this.FileText2 + this.FileText3 + this.FileText4 + this.FileText5 + this.FileText6 + this.FileText7 + this.FileText8 + this.FileText9 + this.FileText10;
                         textData = reader.ReadLine(); 

                        }

                }
                else
                {
                    MessageBox.Show(\"File not found!\");
                }
            }

        }
    }
}

namespace WindowsPhoneApplication1.Model
{
    public class FileItem : ModelBase
    {

        public bool isChecked;
        public bool IsChecked
        {
            get { return this.isChecked; }
            set
            {
                this.isChecked = value;
                this.OnPropertyChanged(\"IsChecked\");
            }
        }    

        public string FileName { get; set; }
        public string FileText { get; set; }
    }

    public class ViewDiskModel : ModelBase
    {
        private IsolatedStorageFile currentStore;
        public IsolatedStorageFile Store
        {
            get
            {
                this.currentStore = this.currentStore ?? IsolatedStorageFile.GetUserStoreForApplication();
                return this.currentStore;
            }
        }

        private ObservableCollection<FileItem> _files;
        public ObservableCollection<FileItem> Files
        {
            get
            {
                this._files = this._files ?? this.LoadFiles();
                return this._files;
            }
        }

        private ObservableCollection<FileItem> LoadFiles()
        {
            ObservableCollection<FileItem> files = new ObservableCollection<FileItem>();

            foreach (string filePath in this.Store.GetFileNames())
                files.Add(new FileItem { FileName = filePath });
            return files;
        }

        private ICommand _deleteSelectedFiles;
        public ICommand DeleteSelectedFiles
        {
            get
            {
                this._deleteSelectedFiles = this._deleteSelectedFiles ?? new DelegateCommand(this.OnDeleteSelected);
                return this._deleteSelectedFiles;
            }
        }

        private ICommand _readSelectedFiles;
        public ICommand ReadSelectedFiles
        {
            get
            {
                this._readSelectedFiles = this._readSelectedFiles ?? new DelegateCommand(this.OnReadSelected);
                return this._readSelectedFiles;
            }
        }

        private void OnDeleteSelected()
        {
            IsolatedStorageFile storage = IsolatedStorageFile.GetUserStoreForApplication();
            List<FileItem> removedItems = new List<FileItem>();
            foreach (var item in this.Files)
            {
                if (item.IsChecked)
                    if (storage.FileExists(item.FileName))
                    {
                        storage.DeleteFile(item.FileName);
                        removedItems.Add(item);
                    }
            }

            foreach (var item in removedItems)
                this.Files.Remove(item);
        }
    }
}

namespace WindowsPhoneApplication1.Model
{
    public class ModelBase : INotifyPropertyChanged
    {
        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventArgs eargs = new PropertyChangedEventArgs(propertyName);
            if (this.PropertyChanged != null)
                this.PropertyChanged(this, eargs);
        }

        #endregion
    }
}

namespace WindowsPhoneApplication1.Model
{
    public class DelegateCommand : ICommand
    {
        private Action command;
        public Action Command
        {
            get { return this.command; }
        }
        private Func<bool> canExecuteCommand;

        public DelegateCommand(Action cmd, Func<bool> canexecute)
        {
            this.canExecuteCommand = canexecute;
            this.command = cmd;
        }

        public DelegateCommand(Action cmd)
        {
            this.command = cmd;
        }

        #region ICommand Members

        public bool CanExecute(object parameter)
        {
            if (this.canExecuteCommand != null)
                return this.canExecuteCommand();

            return true;
        }

        public event EventHandler CanExecuteChanged;

        public void Execute(object parameter)
        {
            if (this.command != null)
                this.command();
        }

        #endregion
    }
}