Thank you for reporting. I couldn't reproduce this (as of yet), but I did make a change to that line of code just now for consistency, tested it and it works fine. I'll change it back to try to reproduce the error as it will make me feel better. What php version are you running?
Just tested it on 7.3.x and still could not reproduce. Attached is a file I made a change to, can you upload it and see if that error goes away?
Just copy it like you would upgrading the add-on, making sure to over-write the file. This zip contains the correct directory structure and only contains the one file (health check will fail for that file, but it's ok).
Good morning, I just re-installed conversation tools 1.3.1 here and hopefully we can reproduce the errors. Can you tell me what settings/permissions you use so I can set them here the same way? I really want to get to the bottom of this asap.
Edit: when do the errors occur? When the pop-up opens to show the list of read and unread conversations, when viewing a conversation, when using Bcc or if not using Bcc? Do they happen all the time?