preview exits with null pointer exception

[kaituo]

[2020-03-23T00:15:33,597][ERROR][c.a.o.a.r.RestExecuteAnomalyDetectorAction] [9801a7a977f1.ant.amazon.com] Unexpected error running anomaly detector jLajBXEBpozDUoJ-TiW2
java.lang.NullPointerException: null
at com.amazon.opendistroforelasticsearch.ad.model.AnomalyResult.toXContent(AnomalyResult.java:94) ~[?:?]
at org.elasticsearch.common.xcontent.XContentBuilder.value(XContentBuilder.java:857) ~[elasticsearch-x-content-7.4.2.jar:7.4.2]
at org.elasticsearch.common.xcontent.XContentBuilder.value(XContentBuilder.java:850) ~[elasticsearch-x-content-7.4.2.jar:7.4.2]
at org.elasticsearch.common.xcontent.XContentBuilder.unknownValue(XContentBuilder.java:828) ~[elasticsearch-x-content-7.4.2.jar:7.4.2]
at org.elasticsearch.common.xcontent.XContentBuilder.value(XContentBuilder.java:915) ~[elasticsearch-x-content-7.4.2.jar:7.4.2]
at org.elasticsearch.common.xcontent.XContentBuilder.unknownValue(XContentBuilder.java:824) ~[elasticsearch-x-content-7.4.2.jar:7.4.2]
at org.elasticsearch.common.xcontent.XContentBuilder.value(XContentBuilder.java:804) ~[elasticsearch-x-content-7.4.2.jar:7.4.2]
at org.elasticsearch.common.xcontent.XContentBuilder.field(XContentBuilder.java:895) ~[elasticsearch-x-content-7.4.2.jar:7.4.2]
at com.amazon.opendistroforelasticsearch.ad.rest.RestExecuteAnomalyDetectorAction$1.lambda$processResponse$0(RestExecuteAnomalyDetectorAction.java:192) ~[?:?]
at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.4.2.jar:7.4.2]
at com.amazon.opendistroforelasticsearch.ad.AnomalyDetectorRunner.lambda$executeDetector$0(AnomalyDetectorRunner.java:67) [opendistro-anomaly-detection-1.4.0.0.jar:1.4.0.0]
at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.4.2.jar:7.4.2]
at com.amazon.opendistroforelasticsearch.ad.feature.FeatureManager.lambda$getPreviewFeatures$17(FeatureManager.java:372) [opendistro-anomaly-detection-1.4.0.0.jar:1.4.0.0]
at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.4.2.jar:7.4.2]
at com.amazon.opendistroforelasticsearch.ad.feature.FeatureManager.lambda$getSamplesForRanges$22(FeatureManager.java:441) [opendistro-anomaly-detection-1.4.0.0.jar:1.4.0.0]
at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.4.2.jar:7.4.2]
at com.amazon.opendistroforelasticsearch.ad.feature.SearchFeatureDao.lambda$getFeatureSamplesForPeriods$17(SearchFeatureDao.java:226) [opendistro-anomaly-detection-1.4.0.0.jar:1.4.0.0]
at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:62) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:70) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:64) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.AbstractSearchAsyncAction.sendSearchResponse(AbstractSearchAsyncAction.java:309) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.FetchSearchPhase$3.run(FetchSearchPhase.java:217) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.AbstractSearchAsyncAction.executePhase(AbstractSearchAsyncAction.java:180) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.AbstractSearchAsyncAction.executeNextPhase(AbstractSearchAsyncAction.java:174) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.ExpandSearchPhase.run(ExpandSearchPhase.java:119) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.AbstractSearchAsyncAction.executePhase(AbstractSearchAsyncAction.java:180) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.AbstractSearchAsyncAction.executeNextPhase(AbstractSearchAsyncAction.java:174) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.FetchSearchPhase.moveToNextPhase(FetchSearchPhase.java:210) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.FetchSearchPhase.lambda$innerRun$2(FetchSearchPhase.java:104) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.FetchSearchPhase.innerRun(FetchSearchPhase.java:110) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.FetchSearchPhase.access$000(FetchSearchPhase.java:44) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.action.search.FetchSearchPhase$1.doRun(FetchSearchPhase.java:86) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.common.util.concurrent.TimedRunnable.doRun(TimedRunnable.java:44) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:773) [elasticsearch-7.4.2.jar:7.4.2]
at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-7.4.2.jar:7.4.2]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
at java.lang.Thread.run(Thread.java:835) [?:?]
[2020-03-23T00:18:12,116][INFO ][c.a.o.j.s.JobSweeper ] [9801a7a977f1.ant.amazon.com] Running full sweep
[2020-03-23T00:23:12,134][INFO ][c.a.o.j.s.JobSweeper ] [9801a7a977f1.ant.amazon.com] Running full sweep
[2020-03-23T00:28:12,163][INFO ][c.a.o.j.s.JobSweeper ] [9801a7a977f1.ant.amazon.com] Running full sweep
[2020-03-23T00:33:12,178][INFO ][c.a.o.j.s.JobSweeper ] [9801a7a977f1.ant.amazon.com] Running full sweep

[kaituo]

need to create a unit test to prevent future regression: In this specific case, the issue occurs in the toXContent method of AnomalyResult class. The test case is just create an input AnomalyResult object that causes the toXContent method to throw NPE before the fix and verify that after the changes, the method does not. A public method is as smallest a unit as there can be.