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journal swamped with gdm3-x-session messages

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.