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:41)
   at org.apache.lucene.index.IndexReader.deleteDocument(IndexReader.java:871)
   at org.netbeans.modules.gsfret.source.usages.LuceneIndex.store(LuceneIndex.java:633)
   at org.netbeans.modules.gsfret.source.usages.SourceAnalyser.analyseUnitAndStore(SourceAnalyser.java:129)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex.runIndexers(PersistentClassIndex.java:167)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex.access$000(PersistentClassIndex.java:84)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex$2$1.run(PersistentClassIndex.java:211)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex$2$1.run(PersistentClassIndex.java:208)
   at org.netbeans.modules.gsfret.source.usages.ClassIndexManager.writeLock(ClassIndexManager.java:110)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex$2.run(PersistentClassIndex.java:207)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex$2.run(PersistentClassIndex.java:204)
   at org.netbeans.napi.gsfret.source.Source.runUserActionTask(Source.java:496)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex.updateDirty(PersistentClassIndex.java:204)
   at org.netbeans.modules.gsfret.source.usages.PersistentClassIndex.search(PersistentClassIndex.java:237)
   at org.netbeans.napi.gsfret.source.ClassIndex.search(ClassIndex.java:121)
   at org.netbeans.modules.python.editor.PythonIndex.search(PythonIndex.java:111)
   at org.netbeans.modules.python.editor.PythonIndex.getModules(PythonIndex.java:135)
   at org.netbeans.modules.python.editor.PythonCodeCompleter.completeObjectMethod(PythonCodeCompleter.java:1387)
   at org.netbeans.modules.python.editor.PythonCodeCompleter.complete(PythonCodeCompleter.java:326)
   at org.netbeans.modules.gsfret.editor.completion.GsfCompletionProvider$JavaCompletionQuery.addCodeCompletionItems(GsfCompletionProvider.java:592)
   at org.netbeans.modules.gsfret.editor.completion.GsfCompletionProvider$JavaCompletionQuery.resolveCompletion(GsfCompletionProvider.java:578)
   at org.netbeans.modules.gsfret.editor.completion.GsfCompletionProvider$JavaCompletionQuery.run(GsfCompletionProvider.java:408)
   at org.netbeans.modules.gsfret.editor.completion.GsfCompletionProvider$JavaCompletionQuery.run(GsfCompletionProvider.java:236)
   at org.netbeans.napi.gsfret.source.Source.runUserActionTask(Source.java:496)
   at org.netbeans.modules.gsfret.editor.completion.GsfCompletionProvider$JavaCompletionQuery.query(GsfCompletionProvider.java:310)
   at org.netbeans.spi.editor.completion.support.AsyncCompletionTask.run(AsyncCompletionTask.java:223)
   at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1424)
   at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:1968)
Caused: org.openide.util.RequestProcessor$FastItem: task failed due to: java.lang.UnsupportedOperationException: This IndexReader cannot make any changes to the index (it was opened with readOnly = true)