FIX FOR 3.1.2 OR NEWER VERSIONS
Some of my colleagues and I have faced the same issue on AS 3.1.2, but a simple Sync didn't help us. For us, the solution was a bit different:
- File -> Invalidate Caches -> Invalidate
- File -> Close Project.
- Remove the project from the AS project selector window.
- Quit from Android Studio
- Start AS and open project again
EXTRA ADVICE IF YOU ARE FACING THIS ISSUE CONSTANTLY
Some of you are constantly facing this issue, and this "solution" fixes it only temporarily. At the time we started getting this error, we used gradle wrapper 4.6 and Android gradle plugin 3.1.0, but since then we went back to gradle 4.4 and we use it with the latest Android gradle plugin, and we haven't seen this issue since then.