I don't know what I did wrong in my Ubuntu 18.04 OS. For some reason some of my apps look like this:
This also happens to other gnome apps like pdf-reviewer and Logs. Window title, menu bar etc. are all blank, display only squares.
I downloaded Google Fonts, saved them in a new folder, they can be applied system-wide, but after I did some font settings using Google Fonts it didn't work. I changed the font of Interface with Tweaks-Tool, that was what causing the problem.