I installed Ubuntu 18.04 right now and after connecting to the internet instead of a network icon there is a question mark. There is no network issues, but that question mark is annoying.
Settings > Privacy > Connectivity Checking
Set it OFF. (Then, restart your WiFi connection, thanks to @Dante's comment below).
This is the same fiasco as Microsoft's NCSI (Network Connectivity Status Indicator), a God damn false negative to try ruin your mood.
And, yeah, it would be checking some URL like that one: http://www.msftncsi.com/ncsi.txt
I've had that issue with other versions of Ubuntu. I had similar Theme issues after upgrading to 18.04.
Change the icon theme. You can do this with Gnome Tweaks.
It's not installed by default. Install it with:
$ apt install gnome-tweak
From there click on Appearance -> (click) Icons. Then select a different theme.
I've seen occasions where changing to a different theme and changing back sometimes resolved a glitch that was happening with the current preferred theme.
I had this issue, along with a very slow internet connection - due to the fact that I had used
expressvpn, and I recently uninstalled it.
The problem was that uninstalling with
apt remove expressvpn didn't clean up after it completely, so I still had a broken
expressvpn service trying to run in
After I ran
apt purge expressvpn, everything started working nicely, and the questionmark over my WiFi icon is gone.