Is see this NullPointerException with Gradle 4.6-rc-1, apply plugin: 'org.akhikhl.gretty'(with classpath 'org.akhikhl.gretty:gretty:2.0.0') and jacoco tool version 0.8.0. The exception is already thrown at configuration time.
No time to craft a minimal reproduction right now, probably next week.