when run article-raking, it thread a NullPointerException.

H:\pgx-samples\article-ranking>gradle run -Pargs="data"

Task :run
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.RuntimeException: java.lang.NullPointerException
at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1908)
at oracle.pgx.api.PgxFuture.get(PgxFuture.java:99)
at oracle.pgx.api.PgxSession.compileProgram(PgxSession.java:2052)
at oracle.pgx.algorithms.ArticleRanker.main(ArticleRanker.java:54)
Caused by: java.lang.RuntimeException: java.lang.NullPointerException
at com.sun.tools.javac.main.Main.compile(Main.java:559)
at com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:129)
at com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:138)
at oracle.pgx.algorithm.AlgorithmCompiler.compileCode(AlgorithmCompiler.java:126)
at oracle.pgx.compilers.algorithm.PgxAlgorithmCompiler.lambda$compileGreenMarl$1(PgxAlgorithmCompiler.java:230)
at oracle.pgx.compilers.Compilers.compile(Compilers.java:84)
at oracle.pgx.compilers.algorithm.PgxAlgorithmCompiler.compileGreenMarl(PgxAlgorithmCompiler.java:228)
at oracle.pgx.compilers.algorithm.PgxAlgorithmCompiler.compile(PgxAlgorithmCompiler.java:205)
at oracle.pgx.compilers.algorithm.PgxAlgorithmCompiler.compile(PgxAlgorithmCompiler.java:62)
at oracle.pgx.engine.invocation.InvocationManagerImpl.compile(InvocationManagerImpl.java:84)
at oracle.pgx.engine.CoreAnalysisImpl$1.doCall(CoreAnalysisImpl.java:62)
at oracle.pgx.engine.CoreAnalysisImpl$1.doCall(CoreAnalysisImpl.java:55)
at oracle.pgx.engine.exec.Task.call(Task.java:326)
at oracle.pgx.engine.exec.Task.run(Task.java:246)
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1067)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1703)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:172)
Caused by: java.lang.NullPointerException
at oracle.pgx.algorithm.plugin.TermBuilder.kindToTerm(TermBuilder.java:294)
at oracle.pgx.algorithm.plugin.TermBuilder.visitMemberReference(TermBuilder.java:290)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitMemberReference(OriginTermBuilder.java:219)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitMemberReference(OriginTermBuilder.java:75)
at com.sun.tools.javac.tree.JCTree$JCMemberReference.accept(JCTree.java:1987)
at oracle.pgx.algorithm.plugin.TermBuilder.lambda$visitMethodInvocation$18(TermBuilder.java:308)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.Iterator.forEachRemaining(Iterator.java:116)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:546)
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:438)
at oracle.pgx.algorithm.plugin.TermBuilder.map(TermBuilder.java:686)
at oracle.pgx.algorithm.plugin.TermBuilder.visitMethodInvocation(TermBuilder.java:308)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitMethodInvocation(OriginTermBuilder.java:226)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitMethodInvocation(OriginTermBuilder.java:75)
at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1477)
at oracle.pgx.algorithm.plugin.TermBuilder.lambda$visitVariable$11(TermBuilder.java:171)
at oracle.pgx.algorithm.plugin.TermBuilder.nullToOption(TermBuilder.java:693)
at oracle.pgx.algorithm.plugin.TermBuilder.visitVariable(TermBuilder.java:170)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitVariable(OriginTermBuilder.java:121)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitVariable(OriginTermBuilder.java:75)
at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:864)
at oracle.pgx.algorithm.plugin.TermBuilder.lambda$visitBlock$13(TermBuilder.java:184)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.Iterator.forEachRemaining(Iterator.java:116)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:546)
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:438)
at oracle.pgx.algorithm.plugin.TermBuilder.map(TermBuilder.java:686)
at oracle.pgx.algorithm.plugin.TermBuilder.visitBlock(TermBuilder.java:184)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitBlock(OriginTermBuilder.java:128)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitBlock(OriginTermBuilder.java:75)
at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:918)
at oracle.pgx.algorithm.plugin.TermBuilder.lambda$visitMethod$7(TermBuilder.java:129)
at oracle.pgx.algorithm.plugin.TermBuilder.nullToOption(TermBuilder.java:693)
at oracle.pgx.algorithm.plugin.TermBuilder.visitMethod(TermBuilder.java:129)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitMethod(OriginTermBuilder.java:100)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitMethod(OriginTermBuilder.java:75)
at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:800)
at oracle.pgx.algorithm.plugin.TermBuilder.lambda$visitClass$1(TermBuilder.java:104)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.Iterator.forEachRemaining(Iterator.java:116)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:546)
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:438)
at oracle.pgx.algorithm.plugin.TermBuilder.map(TermBuilder.java:686)
at oracle.pgx.algorithm.plugin.TermBuilder.visitClass(TermBuilder.java:104)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitClass(OriginTermBuilder.java:93)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitClass(OriginTermBuilder.java:75)
at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:720)
at oracle.pgx.algorithm.plugin.TermBuilder.lambda$visitCompilationUnit$0(TermBuilder.java:91)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.Iterator.forEachRemaining(Iterator.java:116)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:546)
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:438)
at oracle.pgx.algorithm.plugin.TermBuilder.map(TermBuilder.java:686)
at oracle.pgx.algorithm.plugin.TermBuilder.visitCompilationUnit(TermBuilder.java:91)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitCompilationUnit(OriginTermBuilder.java:86)
at oracle.pgx.algorithm.plugin.OriginTermBuilder.visitCompilationUnit(OriginTermBuilder.java:75)
at com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:550)
at com.sun.source.util.TreeScanner.scan(TreeScanner.java:77)
at oracle.pgx.algorithm.plugin.GreenMarlTaskListener.javaTreeToJavaTerm(GreenMarlTaskListener.java:299)
at oracle.pgx.algorithm.plugin.GreenMarlTaskListener.run(GreenMarlTaskListener.java:137)
at oracle.pgx.algorithm.util.PostAnalyzeTaskListener.finished(PostAnalyzeTaskListener.java:40)
at com.sun.tools.javac.api.ClientCodeWrapper$WrappedTaskListener.finished(ClientCodeWrapper.java:681)
at com.sun.tools.javac.api.MultiTaskListener.finished(MultiTaskListener.java:111)
at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1342)
at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1296)
at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:901)
at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:860)
at com.sun.tools.javac.main.Main.compile(Main.java:523)
... 18 more

Task :run FAILED

FAILURE: Build failed with an exception.

• What went wrong:
  Execution failed for task ':run'.

Process 'command 'G:\self\java\jdk1.8\bin\java.exe'' finished with non-zero exit value 1

• Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

• Get more help at https://help.gradle.org

BUILD FAILED in 13s
3 actionable tasks: 2 executed, 1 up-to-date

Reproduce the failure with gradle run -Pargs="data" and inspect oracle.pgx.algorithms.ArticleRanker.main at ArticleRanker.java:54. Trace the compiler path through TermBuilder.visitMemberReference at TermBuilder.java:290-294, using the stack trace to identify the unsupported or missing member-reference term. Done means the article-ranking sample runs without this NullPointerException.