Cannot resolve symbol Theme, ThemeOverlay

Question

I updated my android studio to latest version - 3.0.1. Since then it complains \'Cannot resolve symbol\' for Theme and ThemeOverlay in the following lines of code. App runs

Answer 1

Something got messed up on the indexing of the Project when you updated the version.

To fix go to the menu:

File -> Invalidate Caches/Restart

that will reopen and re-index the whole project, the error should be gone.