Android Studio 3.1: Erroneous unresolved references in editor

1. Exit Studio.
2. Delete .idea/ (seems to me that's most important), build/, app/build directories.
3. Start Studio
4. Set Settings -> Build -> Gradle settings back (last stable Gradle local distribution in my case).
5. Clean&rebuild project.

Helps me.

Delete {projectDir}/.idea/libraries, then go to File -> Sync Project with Gradle Files.