After updating Android Studio and Gradle to 3.5, I now get this error:
NDK Resolution Outcome: Project settings: Gradle model version=5.4.1, NDK version is UNKNOWN
I changed the Gradle version in build-gradle back to 3.4.2 but it didn't help.
I had similar problem and solved it by opening project using Import project (Gradle, Eclipse, etc.)
instead of Open existing Android Studio project
.