Hi and thanks for the heads-up and the patch. After a long time, I have finally had the time to try using the latest GIMP development versions, but I can't see any of the error messages you mention. Furthermore, I'm afraid that applying the patch will break compatibility with GIMP 2.6, so I'm keeping this in mind but I'm not adopting the new API for the time being.
Let me know if the problem is still present.