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.