public VersionOnlyBuildEnvironment(String gradleVersion) {
        this.gradleVersion = gradleVersion;
    }

    public VersionOnlyGradleEnvironment getGradle() {
        return new VersionOnlyGradleEnvironment();
    }

    public class VersionOnlyGradleEnvironment {
        public String getGradleVersion() {
            return gradleVersion;
        }
    }

Class <org.gradle.tooling.internal.build.VersionOnlyBuildEnvironment$VersionOnlyGradleEnvironment> is not annotated (directly or via its package) with @org.gradle.api.NonNullApi in (VersionOnlyBuildEnvironment.java:0)