It seems that the correct fix for this problem is to simply remove the file /etc/X11/Xsession.d/99mate-environment altogether. It’s been replaced in 18.04 with 80mate-environment, which should also be present in the same directory and 99mate-environment should have been removed in the upgrade. Look at the file dates on these two files and you’ll see that the former dates from 2016, while the latter replacement file dates from 2018. See Problems with the dropbox icon in the panel (UM 18.04.1) - Look down the page a way for the comment on this by Norbert_X, Aug. 18. This solution worked for me.
I expect that this is a bug in the upgrade process.