Magento 2 getting error “Broken reference: The element with the “customer” ID wasn’t found. Verify the ID and try again. [] []” in system.log

I am getting lots of

Broken reference: The element with the "customer" ID wasn't found. Verify the ID and try again. [] [] 

errors in the error.log. I’ve googled around and tried to trace it with NetBeans but I can’t track down what’s causing it. Everything seems to be working perfectly except this error is getting written repeatedly.

Has anyone any idea what’s causing it or how I might track it down?

As some background, I migrated my site from 1.8.1 to 2.3 (and have since upgraded to 2.3.1 but the error started in 2.3). I wonder if something hasn’t migrated correctly. Note it occurs in both developer and production mode.