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

Example

setContentView(R.layout.activity_login_ativity);

1. Put the cursor in R
2. Click Alt+Enter
3. Choose import class R

Work for me :)