NPE on GradlePlugin

Gradle sonar plugin 3.1.1 ends up in an NPE without any message.
runs with ./gradlew clean build sonarqube --debug --stacktrace local and on Github actions

Caused by: java.lang.NullPointerException
at org.sonar.scanner.report.ComponentsPublisher.convert(ComponentsPublisher.java:89)
at org.sonar.scanner.report.ComponentsPublisher.publish(ComponentsPublisher.java:54)
at org.sonar.scanner.report.ReportPublisher.generateReportFile(ReportPublisher.java:163)
at org.sonar.scanner.report.ReportPublisher.execute(ReportPublisher.java:136)
at org.sonar.scanner.scan.ProjectScanContainer.doAfterStart(ProjectScanContainer.java:404)
at org.sonar.core.platform.ComponentContainer.startComponents(ComponentContainer.java:123)
at org.sonar.core.platform.ComponentContainer.execute(ComponentContainer.java:109)
at org.sonar.scanner.bootstrap.GlobalContainer.doAfterStart(GlobalContainer.java:126)
at org.sonar.core.platform.ComponentContainer.startComponents(ComponentContainer.java:123)
at org.sonar.core.platform.ComponentContainer.execute(ComponentContainer.java:109)
at org.sonar.batch.bootstrapper.Batch.doExecute(Batch.java:58)
at org.sonar.batch.bootstrapper.Batch.execute(Batch.java:52)
at org.sonarsource.scanner.api.internal.batch.BatchIsolatedLauncher.execute(BatchIsolatedLauncher.java:46)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)