why my wifi logo have an Exclamation mark on ubuntu
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.