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

Just importing project again did not work for me.

My solution was

1. Delete .idea folder inside your project.
2. Then close project by File> Close Project
3. Then import project by File> New > Import Project OR from welcome Screen Import Project (Gradle, Eclipse ADT, etc.).

See screenshot for more info

Or