calibre, version 0.7.55 ERROR: Unhandled exception: <b>TypeError</b>:object of type 'NoneType' has no len() Traceback (most recent call last): File "calibre_plugins.find_duplicates.action", line 143, in find_duplicates File "calibre_plugins.find_duplicates.duplicates", line 318, in run_duplicate_check TypeError: object of type 'NoneType' has no len()