Hmm well the code causing the error seems to happen after STM calls ForceUpdateCanvases, so maybe the canvas registry is being told to update a canvas it doesn't know about yet...?
Is your call to set text on Awake()? Switching it to Start() vould jave different behaviour.
That said, I think I can come up with a better way to make the canvas update, so I want to try that, too. I'll try to reproduce this momentarily!
Edit: I'm trying to reproduce this now, and I can't seem to... (Calling FindObjectsOfType() and setting all text meshes upon Awake()) Can I see a bit of your code to get a better idea of when this is being called? Thank you!