journal swamped with gdm3-x-session messages

[Roger Price]

I rebooted stretch and now my journald is being swamped with the following 
message:

 /usr/lib/gdm3/gdm-x-session[2684]: Promise rejected after context unloaded:
 Message manager disconnected
A new message appears every few seconds.  What is causing this?
What have I done wrong?  Any hint would be much appreciated.

Roger