It appears the ExternalizerAdapter was not added to the readers, or that input.readUnsignedByte() reads a wrong identifier value.
Not easy to reproduce, get this sometimes under load:

Exception in thread "Scarlet/Lucene service thread index reopening thread-1" java.lang.NullPointerException
at org.infinispan.marshall.jboss.ConstantObjectTable.readObject(ConstantObjectTable.java:253)
at org.jboss.marshalling.river.RiverUnmarshaller.doReadObject(RiverUnmarshaller.java:357)
at org.jboss.marshalling.river.RiverUnmarshaller.doReadObject(RiverUnmarshaller.java:207)
at org.jboss.marshalling.AbstractUnmarshaller.readObject(AbstractUnmarshaller.java:85)
at org.infinispan.marshall.jboss.GenericJBossMarshaller.objectFromObjectStream(GenericJBossMarshaller.java:162)
at org.infinispan.marshall.VersionAwareMarshaller.objectFromByteBuffer(VersionAwareMarshaller.java:111)
at org.infinispan.marshall.AbstractStreamingMarshaller.objectFromInputStream(AbstractStreamingMarshaller.java:23)
at org.infinispan.loaders.jdbc.JdbcUtil.unmarshall(JdbcUtil.java:88)
at org.infinispan.loaders.jdbc.stringbased.JdbcStringBasedCacheStore.loadLockSafe(JdbcStringBasedCacheStore.java:298)
at org.infinispan.loaders.LockSupportCacheStore.load(LockSupportCacheStore.java:100)
at org.infinispan.loaders.decorators.AbstractDelegatingStore.load(AbstractDelegatingStore.java:86)
at org.infinispan.interceptors.CacheLoaderInterceptor.loadIfNeeded(CacheLoaderInterceptor.java:138)
at org.infinispan.interceptors.CacheLoaderInterceptor.loadIfNeededAndUpdateStats(CacheLoaderInterceptor.java:200)
at org.infinispan.interceptors.CacheLoaderInterceptor.visitGetKeyValueCommand(CacheLoaderInterceptor.java:87)
at org.infinispan.commands.read.GetKeyValueCommand.acceptVisitor(GetKeyValueCommand.java:58)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.base.CommandInterceptor.handleDefault(CommandInterceptor.java:132)
at org.infinispan.commands.AbstractVisitor.visitGetKeyValueCommand(AbstractVisitor.java:87)
at org.infinispan.commands.read.GetKeyValueCommand.acceptVisitor(GetKeyValueCommand.java:58)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.TxInterceptor.enlistReadAndInvokeNext(TxInterceptor.java:171)
at org.infinispan.interceptors.TxInterceptor.visitGetKeyValueCommand(TxInterceptor.java:162)
at org.infinispan.commands.read.GetKeyValueCommand.acceptVisitor(GetKeyValueCommand.java:58)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.CacheMgmtInterceptor.visitGetKeyValueCommand(CacheMgmtInterceptor.java:83)
at org.infinispan.commands.read.GetKeyValueCommand.acceptVisitor(GetKeyValueCommand.java:58)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.InvocationContextInterceptor.handleAll(InvocationContextInterceptor.java:57)
at org.infinispan.interceptors.InvocationContextInterceptor.handleDefault(InvocationContextInterceptor.java:38)
at org.infinispan.commands.AbstractVisitor.visitGetKeyValueCommand(AbstractVisitor.java:87)
at org.infinispan.commands.read.GetKeyValueCommand.acceptVisitor(GetKeyValueCommand.java:58)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.BatchingInterceptor.handleDefault(BatchingInterceptor.java:76)
at org.infinispan.commands.AbstractVisitor.visitGetKeyValueCommand(AbstractVisitor.java:87)
at org.infinispan.commands.read.GetKeyValueCommand.acceptVisitor(GetKeyValueCommand.java:58)
at org.infinispan.interceptors.InterceptorChain.invoke(InterceptorChain.java:273)
at org.infinispan.CacheDelegate.get(CacheDelegate.java:200)
at org.infinispan.lucene.FileListOperations.getFileList(FileListOperations.java:54)