NPE on Travis when running tests in JarFilePackageLister.java:53

The Travis build of the open-source ELKI project began failing, without source change. So it probably is caused by some dependency changed. And it builds fine on my machine, so I don’t know how to debug this.

Log with --stacktrace: https://travis-ci.org/elki-project/elki/builds/312108363#L3621

Caused by: java.lang.NullPointerException
	at org.gradle.api.internal.tasks.testing.detection.JarFilePackageLister.listJarPackages(JarFilePackageLister.java:53)
	at org.gradle.api.internal.tasks.testing.detection.ClassFileExtractionManager.addLibraryJar(ClassFileExtractionManager.java:55)
	at org.gradle.api.internal.tasks.testing.detection.AbstractTestFrameworkDetector.prepareClasspath(AbstractTestFrameworkDetector.java:107)
	at org.gradle.api.internal.tasks.testing.detection.AbstractTestFrameworkDetector.getSuperTestClassFile(AbstractTestFrameworkDetector.java:65)
	at org.gradle.api.internal.tasks.testing.junit.JUnitDetector.processTestClass(JUnitDetector.java:50)
	at org.gradle.api.internal.tasks.testing.detection.AbstractTestFrameworkDetector.processTestClass(AbstractTestFrameworkDetector.java:142)
	at org.gradle.api.internal.tasks.testing.detection.DefaultTestClassScanner$1.visitClassFile(DefaultTestClassScanner.java:57)
	at org.gradle.api.internal.tasks.testing.detection.DefaultTestClassScanner$ClassFileVisitor.visitFile(DefaultTestClassScanner.java:78)
	at org.gradle.api.internal.file.collections.jdk7.Jdk7DirectoryWalker$1.visitFile(Jdk7DirectoryWalker.java:86)
	at org.gradle.api.internal.file.collections.jdk7.Jdk7DirectoryWalker$1.visitFile(Jdk7DirectoryWalker.java:59)
	at org.gradle.api.internal.file.collections.jdk7.Jdk7DirectoryWalker.walkDir(Jdk7DirectoryWalker.java:59)
	at org.gradle.api.internal.file.collections.DirectoryFileTree.walkDir(DirectoryFileTree.java:155)
	at org.gradle.api.internal.file.collections.DirectoryFileTree.visitFrom(DirectoryFileTree.java:133)
	at org.gradle.api.internal.file.collections.DirectoryFileTree.visit(DirectoryFileTree.java:118)
	at org.gradle.api.internal.file.collections.FileTreeAdapter.visit(FileTreeAdapter.java:110)
	at org.gradle.api.internal.file.CompositeFileTree.visit(CompositeFileTree.java:87)
	at org.gradle.api.internal.tasks.testing.detection.DefaultTestClassScanner.detectionScan(DefaultTestClassScanner.java:55)
	at org.gradle.api.internal.tasks.testing.detection.DefaultTestClassScanner.run(DefaultTestClassScanner.java:49)
	at org.gradle.api.internal.tasks.testing.processors.TestMainAction.run(TestMainAction.java:55)
	at org.gradle.api.internal.tasks.testing.detection.DefaultTestExecuter.execute(DefaultTestExecuter.java:103)
	at org.gradle.api.tasks.testing.Test.executeTests(Test.java:679)
	at org.gradle.internal.reflect.JavaMethod.invoke(JavaMethod.java:73)
	at org.gradle.api.internal.project.taskfactory.StandardTaskAction.doExecute(StandardTaskAction.java:46)
	at org.gradle.api.internal.project.taskfactory.StandardTaskAction.execute(StandardTaskAction.java:39)
	at org.gradle.api.internal.project.taskfactory.StandardTaskAction.execute(StandardTaskAction.java:26)
	at org.gradle.api.internal.AbstractTask$TaskActionWrapper.execute(AbstractTask.java:784)
	at org.gradle.api.internal.AbstractTask$TaskActionWrapper.execute(AbstractTask.java:751)
	at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter$1.run(ExecuteActionsTaskExecuter.java:121)
	at org.gradle.internal.progress.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:336)
	at org.gradle.internal.progress.DefaultBuildOperationExecutor$RunnableBuildOperationWorker.execute(DefaultBuildOperationExecutor.java:328)
	at org.gradle.internal.progress.DefaultBuildOperationExecutor.execute(DefaultBuildOperationExecutor.java:199)
	at org.gradle.internal.progress.DefaultBuildOperationExecutor.run(DefaultBuildOperationExecutor.java:110)
	at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeAction(ExecuteActionsTaskExecuter.java:110)
	at org.gradle.api.internal.tasks.execution.ExecuteActionsTaskExecuter.executeActions(ExecuteActionsTaskExecuter.java:92)
	... 29 more