Android Studio 3.1 Cannot Resolve Symbol (Themes, Widget, attr, etc.)

Question

I upgraded Android Studio today to 3.1, and now Android Studio says it cannot resolve symbols for most of the resources (for example ThemeOverlay in style

Answer 1

You can try to close the project and exit Android Studio (only closing and reimporting it didn't do it for me). Launch Android Studio and open the project again. That worked for me!