I have a problem using StorageApplicationPermissions both to write data and read data

My plan is to write serializable data then read it again. I keep getting the exception error (see the code sequence below) reporting:-

Error: cant' find Folder! I am sure I must be doing something quite basically wrong. Any guidance from more informed developers would be most gratefully received.

    static public string FolderAccessToken { get; set; }
    static public async void SelectFolder()
    {
        try
        {

            var picker = new FolderPicker();
            picker.FileTypeFilter.Add(".xml");
            //picker.SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.DocumentsLibrary;
            var folder = await picker.PickSingleFolderAsync();
            if (folder == null)
            {
                FolderAccessToken = "";
                return;
            }

            // Get the future access token by adding the StorageFolder object to the FutureAccessList, 
            // then save the token for future use (it can be re-used after quitting and re-starting app)
            FolderAccessToken = StorageApplicationPermissions.FutureAccessList.Add(folder);

            if (string.IsNullOrEmpty(FolderAccessToken))
            {
                datagramsocket.message_to_send = "Error: can't create or find token!";
                return;
            }

            // Save the token in the app's roaming settings
            ApplicationData.Current.LocalSettings.Values["DocumentsLibraryAccessToken"] = FolderAccessToken;
        }
        catch (Exception e)
        {
            datagramsocket.message_to_send = "innerexception in SelectFolder()b: " + e.InnerException;
            FolderAccessToken = "";
            return;
        }
        datagramsocket.message_to_send = "token: " + FolderAccessToken;
    }
    static public async  void WriteToXML()
    {
        try
        {
            // Get the user to select the Documents library folder (or a sub-folder),
            // thereby granting you access permissions...
            if (ApplicationData.Current.LocalSettings.Values.ContainsKey("FolderAccessToken"))
                FolderAccessToken = (string)ApplicationData.Current.LocalSettings.Values["FolderAccessToken"];
            else
            {
                SelectFolder();
            }
            //check if it is still null or empty!
            if (string.IsNullOrEmpty(FolderAccessToken))
            {
                datagramsocket.message_to_send = "WriteToDocLib Error: can't find Folder!";
                return;
            }

            // Get a StorageFolder object using the DocumentsLibraryAccessToken

            var folder =await  StorageApplicationPermissions.FutureAccessList.GetFolderAsync(FolderAccessToken);

            // Check we still have access
            var accessAllowed = StorageApplicationPermissions.FutureAccessList.CheckAccess(folder);
            if (!accessAllowed)
            {
                SelectFolder();
                if (string.IsNullOrEmpty(FolderAccessToken))
                {
                    datagramsocket.message_to_send = "Error: can't create or find token!";
                    return;
                }
            }
            var subfolder = await folder.GetFolderAsync(foldername);
            SerializeObject(subfolder, xmlfile);
            datagramsocket.message_to_send = "serialized to: " + xmlfile;
        }
        catch(Exception e)
        {
            datagramsocket.message_to_send = "ex in WriteToDocLib: " + e.Message;
        }
    }

These statics just live in a useful class I have developed. The phrase: datagramsocket.message_to_send = ... sends the data to a Broadcast receiver on my PC exaclty the same as in the Windows Universal Samples suite that can be downloaded to check UWP Apps. Fundamentally, I try to check for a valid token already and if one is not available, try to create one.