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.netbeans.modules.gsfret.source.usages.LuceneIndex.store(LuceneIndex.java:633)
   at org.netbeans.modules.gsfret.source.usages.SourceAnalyser.delete(SourceAnalyser.java:141)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker.delete(RepositoryUpdater.java:1634)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker.access$2200(RepositoryUpdater.java:654)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$CompileWorker$1.run(RepositoryUpdater.java:931)
   at org.netbeans.modules.gsfret.source.usages.RepositoryUpdater$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$RunnableAdapter.call(Executors.java:471)
   at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
   at java.util.concurrent.FutureTask.run(FutureTask.java:166)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
   at java.lang.Thread.run(Thread.java:722)