Got around to doing more testing with the updated plugin and it spits out an error message when I attempt to modify more that one file at a time. My test was with 3 files and got the error message after clicking Yes to replace the versions in the library (the log file prior to clicking yes did not show any errors). Doing those 3 files one at a time did not generate the error message.
Code:
calibre, version 8.0.1
ERROR: Unhandled exception: <b>AttributeError</b>:'NoneType' object has no attribute 'update_last_modified'
...
Traceback (most recent call last):
File "calibre_plugins.modify_epub.dialogs", line 291, in do_book_check
File "calibre_plugins.modify_epub.dialogs", line 310, in do_close
AttributeError: 'NoneType' object has no attribute 'update_last_modified'