I read about this error before but never faced it, until today.
I ran a
sudo apt-get update && sudo apt-get upgrade && sudo apt-get dist-upgrade to make my lap up to date. and ended up with to problems. Hopefully I can Fix them. 1. Pantheon-Mail now stretches horizontally across the screen. 2. when I click “network settings” from drop-down menu of network icon on wing panel, it opens “system settings”.
after a brief googling I found solution for the first one.
kill or close pantheon-mail then run the following command on terminal.
dconf reset -f /org/pantheon/mail/
Which will reset the settings of Mail to default. For me after running this it was solved.
(some user said this is not a permanent solution and i may have to run the same when it appears again! )