Since this hasn't been updated yet, I want to go ahead and add this bit of discussion, as it is relevant to the problem.

Daimona's patch worked, and I was able to continue my upgrade for both of my wiki sites. I ran into this error originally while upgrading from 1.29 to 1.35, and I saw it with both sites. I would like to know if this could potentially introduce database corruption down the line, however.

Just like to note that I encountered this error while upgrading from 1.29.1 to 1.35.0, and I was able to work around it by running the maintenance script populateContentTables.php, which failed with a duplicate entry for key "PRIMARY" error but still allowed the update.php script to continue. It seems like the updater tried to work on the "slots" table before it was even populated, hence the slot not found error.