NullPointerException during retention

[hc4]

Just got error in logs:

2016-08-08T13:33:08.495+03:00 INFO  [AbstractIndexCountBasedRetentionStrategy] Number of indices (133) higher than limit (130). Running retention for 3 indices.
2016-08-08T13:33:08.559+03:00 INFO  [AbstractIndexCountBasedRetentionStrategy] Running retention strategy [org.graylog2.indexer.retention.strategies.DeletionRetentionStrategy] for index <graylog2_96>
2016-08-08T13:33:09.406+03:00 ERROR [AnyExceptionClassMapper] Unhandled exception in REST resource
java.lang.NullPointerException
        at org.graylog2.indexer.indices.Indices.checkForReopened(Indices.java:382) ~[graylog.jar:?]
        at org.graylog2.indexer.indices.Indices.lambda$areReopened$1(Indices.java:377) ~[graylog.jar:?]
        at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1321) ~[?:1.8.0_91]
        at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169) ~[?:1.8.0_91]
        at java.util.HashMap$KeySpliterator.forEachRemaining(HashMap.java:1540) ~[?:1.8.0_91]
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) ~[?:1.8.0_91]
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[?:1.8.0_91]
        at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[?:1.8.0_91]
        at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:1.8.0_91]
        at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[?:1.8.0_91]
        at org.graylog2.indexer.indices.Indices.areReopened(Indices.java:376) ~[graylog.jar:?]
        at org.graylog2.rest.resources.system.indexer.IndexerOverviewResource.index(IndexerOverviewResource.java:83) ~[graylog.jar:?]
        at sun.reflect.GeneratedMethodAccessor136.invoke(Unknown Source) ~[?:?]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_91]
        at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_91]
        at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory$1.invoke(ResourceMethodInvocationHandlerFactory.java:81) ~[graylog.jar:?]
        at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher$1.run(AbstractJavaResourceMethodDispatcher.java:144) ~[graylog.jar:?]
        at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:161) ~[graylog.jar:?]
        at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$TypeOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:205) ~[graylog.jar:?]
        at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:99) ~[graylog.jar:?]
        at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:389) ~[graylog.jar:?]
        at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:347) ~[graylog.jar:?]
        at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:102) ~[graylog.jar:?]
        at org.glassfish.jersey.server.ServerRuntime$2.run(ServerRuntime.java:326) [graylog.jar:?]
        at org.glassfish.jersey.internal.Errors$1.call(Errors.java:271) [graylog.jar:?]
        at org.glassfish.jersey.internal.Errors$1.call(Errors.java:267) [graylog.jar:?]
        at org.glassfish.jersey.internal.Errors.process(Errors.java:315) [graylog.jar:?]
        at org.glassfish.jersey.internal.Errors.process(Errors.java:297) [graylog.jar:?]
        at org.glassfish.jersey.internal.Errors.process(Errors.java:267) [graylog.jar:?]
        at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:317) [graylog.jar:?]
        at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:305) [graylog.jar:?]
        at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:1154) [graylog.jar:?]
        at org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpContainer.service(GrizzlyHttpContainer.java:384) [graylog.jar:?]
        at org.glassfish.grizzly.http.server.HttpHandler$1.run(HttpHandler.java:224) [graylog.jar:?]
        at com.codahale.metrics.InstrumentedExecutorService$InstrumentedRunnable.run(InstrumentedExecutorService.java:176) [graylog.jar:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [?:1.8.0_91]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [?:1.8.0_91]
        at java.lang.Thread.run(Thread.java:745) [?:1.8.0_91]
2016-08-08T13:33:14.292+03:00 INFO  [DeletionRetentionStrategy] Finished index retention strategy [delete] for index <graylog2_96> in 5732ms.

Looks like some race-condition.
Not sure if it is critical

Your Environment

• Graylog Version: 2.1.0-beta2