Using TextEdit, I did a search for “NoneNone” in the .uasset blueprint files I had been working on recently. Sure enough, it was there.
The culprit was an object reference which I had duplicated. I resolved the issue by renaming both variables (the original and the duplicate). I did not have to delete anything. I just renamed both variables and the “NoneNone” error is gone.
Thanks for the help on this!