If you have created a reading list, added it to the Borrower Portal widget and then deleted the list but left the widget live, you will be presented with an error message to make you aware of the missing list.
Non-admin users or when not logged in:
Admin users when logged in:
You can also see a warning on the configure settings:
To resolve this, you will need to delete the widget from the Borrower Portal, you can then add it again with a new reading list.