Try the following
- Rebuilding the project
- Cleaning the project
- Syncing with Gradle Files
- Closing the Project, closing AS and relaunching / reopening
- File > Invalidate Caches / Restart
- Double checking all support libraries are up to date in the SDK manager
- Exit Android Studio
- Back up your project
- Delete all the .iml files and the .idea folder, build folder
- Relaunch Android Studio and reimport your project
