This is while adding a book to a shelf? The changes the beta will not have affected this, but it will mean I can track what the error reports against the code. Unfortunately, it doesn't tell me much more than before.
Can you run calibre in debug mode and post the log? Right click on the preferences button and select "Restart in debug mode". Then do whatever it is that causes the problem and close calibre. The debug log will be displayed. Post the complete log. Hopefully it will tell me something about what is happening.
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