java.lang.UnsupportedOperationException: This IndexReader cannot make any changes to the index (it was opened with readOnly = true)
   at org.apache.lucene.index.ReadOnlySegmentReader.noWrite(ReadOnlySegmentReader.java:23)
   at org.apache.lucene.index.ReadOnlyDirectoryReader.acquireWriteLock(ReadOnlyDirectoryReader.java:43)
   at org.apache.lucene.index.IndexReader.deleteDocument(IndexReader.java:1339)
   at org.apache.lucene.index.IndexReader.deleteDocuments(IndexReader.java:1375)
   at org.netbeans.modules.gsfret.source.usages.LuceneIndex.batchStore(LuceneIndex.java:515)
   at org.netbeans.modules.gsfret.source.usages.SourceAnalyser.batchStore(SourceAnalyser.java:171)
   at org.netbeans.modules.gsfret.source.usages.CachingIndexer$LanguageIndex.flush(CachingIndexer.java:159)
   at org.netbeans.modules.gsfret.source.usages.CachingIndexer.flush(CachingIndexer.java:112)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker.updateFolder(RepositoryUpdater.java:1414)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker.scanRoots(RepositoryUpdater.java:1132)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker.access$1900(RepositoryUpdater.java:654)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker$1.run(RepositoryUpdater.java:841)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker$1.run(RepositoryUpdater.java:679)
   at org.netbeans.modules.gsfret.source.usages.ClassIndexManager.writeLock(ClassIndexManager.java:110)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker.run(RepositoryUpdater.java:679)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker.run(RepositoryUpdater.java:654)
   at org.netbeans.napi.gsfret.source.Source$CompilationJob.run(Source.java:1358)
   at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   at java.lang.Thread.run(Thread.java:745)