[ownedbycats]

I ran the chain once successfully with two books. Then I cleared the books. The next time I ran it (on one book) it showed this error:

calibre, version 5.5.0
ERROR: Chain Error: Validating the chain settings before running failed.                You can see the detailed errors by opening the chain in the config dialog

Opening it showed this:

Calibre plugin "Mark Books" cannot be found

Also: I got this error twice when trying to change one action to another (calibre actions > search modifiers, and then vice versa) instead of deleting it and making a new one:

calibre, version 5.5.0
ERROR: Unhandled exception: <b>TypeError</b>:string indices must be integers

calibre 5.5 [64bit]  embedded-python: True is64bit: True
Windows-10-10.0.19041 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.19041')
Python 3.8.5
Windows: ('10', '10.0.19041', '', 'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: Action Chains (0, 1, 0) && Barnes & Noble (1, 2, 16) && Calibre Power Search Plugin (1, 7, 0) && CalibreSpy (1, 0, 83) && Comments Cleaner (1, 3, 1) && Count Pages (1, 11, 0) && Drop Search Results (1, 0, 10) && Embed Comic Metadata (1, 5, 2) && EpubCheck (0, 2, 3) && EpubMerge (2, 11, 0) && Extract ISBN (1, 5, 0) && FanFicFare (3, 25, 8) && Fantastic Fiction (1, 4, 0) && Favourites Menu (1, 0, 5) && Find Duplicates (1, 8, 3) && Generate Cover (2, 0, 0) && GetFileName (0, 1, 1) && Goodreads (1, 5, 1) && Import List (1, 5, 1) && Job Spy (1, 0, 188) && Kindle hi-res covers (0, 5, 0) && Kobo Books (1, 8, 0) && Kobo Utilities (2, 11, 8) && KoboTouchExtended (3, 4, 1) && Library Codes (1, 0, 48) && Manage Series (1, 2, 11) && Modify ePub (1, 6, 1) && MultiColumnSearch (1, 0, 85) && Open With (1, 5, 13) && Quality Check (1, 10, 1) && Reading List (1, 7, 1) && Save To Format (0, 0, 6) && Search The Internet (1, 8, 0) && SmartEject (2, 3, 0) && Sum Column (0, 3, 1) && View Manager (1, 6, 0)
Traceback (most recent call last):
  File "calibre_plugins.action_chains.dialogs.chain", line 206, in _settings_button_clicked
  File "calibre_plugins.action_chains.dialogs.chain", line 53, in load_settings
  File "calibre_plugins.action_chains.actions.calibre_actions", line 241, in load_settings
  File "calibre_plugins.action_chains.actions.calibre_actions", line 235, in _repopulate_actions_tree
  File "calibre_plugins.action_chains.actions.calibre_actions", line 88, in _build_lookup_menu_map
TypeError: string indices must be integers