Comment 16 for bug 1425349

Upstream has fixed that problem long time ago.
https://mail.gnome.org/archives/commits-list/2014-September/msg04244.html

As Xenial now only receive 'security' updates, but use mutter 3.18.x which should include the fix, if your system still log that error, then its time to clean the system and/or reconfigure mutter & associated packages. At last recent ubuntu versions are not spammed with this problem.