fixed advanced demo not turning pages by returning fallback value if index is invalid in _getDataProp
fixed advanced demo not turning pages by returning fallback value if index is invalid in _getDataProp

openlibrary/bookreader,openlibrary/bookreader,openlibrary/bookreader,internetarchive/bookreader,internetarchive/bookreader,openlibrary/bookreader,openlibrary/bookreader