Hololens2でKnownFolders.DocumentsLibraryを扱おうとしたら「index out of range of ToggleList: 0」エラーが出た

string UWP_DIR_NAME = "myDir";
string FILE_NAME = "obj_info.json";

string jsonString = "{\"objs\":{\"x\":100,\"y\":200,\"z\":300}";

await Task.Run(async () =>
{
    try
    {
        var readFolder = await KnownFolders.DocumentsLibrary.GetFolderAsync(UWP_DIR_NAME);
        var readFile = await readFolder.GetFileAsync(FILE_NAME);
        using (var stream = await readFile.OpenStreamForReadAsync())
        {
            var bytes = new byte[stream.Length];
            await stream.ReadAsync(bytes, 0, bytes.Length);
            var text = System.Text.Encoding.UTF8.GetString(bytes);
            jsonString = text;
        }
    }
    catch (Exception e)
    {
        var folder = await KnownFolders.DocumentsLibrary.CreateFolderAsync(UWP_DIR_NAME, CreationCollisionOption.OpenIfExists);
        var file = await folder.CreateFileAsync(FILE_NAME, CreationCollisionOption.ReplaceExisting);
        using (var stream = await file.OpenStreamForWriteAsync())
        {
            var bytes = System.Text.Encoding.UTF8.GetBytes(jsonString);
            await stream.WriteAsync(bytes, 0, bytes.Length);
        }
    }
});