java.lang.NullPointerException
        at org.netbeans.modules.java.source.save.CasualDiff.isCommaSeparated(CasualDiff.java:2623)
        at org.netbeans.modules.java.source.save.CasualDiff.filterHidden(CasualDiff.java:1851)
        at org.netbeans.modules.java.source.save.CasualDiff.diffClassDef(CasualDiff.java:328)
        at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:2195)
        at org.netbeans.modules.java.source.save.CasualDiff.diffListImports(CasualDiff.java:1956)
        at org.netbeans.modules.java.source.save.CasualDiff.diffTopLevel(CasualDiff.java:143)
        at org.netbeans.modules.java.source.save.CasualDiff.diffTree(CasualDiff.java:2189)
        at org.netbeans.modules.java.source.save.CasualDiff.diff(CasualDiff.java:91)
        at org.netbeans.modules.java.source.save.Commit.commit(Commit.java:227)
        at org.netbeans.modules.java.source.save.Commit.visitCompilationUnit(Commit.java:162)
        at org.netbeans.modules.java.source.save.Commit.visitCompilationUnit(Commit.java:58)
        at com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:476)
        at org.netbeans.modules.java.source.engine.RootTree.accept(RootTree.java:60)
        at org.netbeans.modules.java.source.query.Query.apply(Query.java:95)
        at org.netbeans.modules.java.source.save.Commit.commit(Commit.java:105)
        at org.netbeans.api.java.source.WorkingCopy.getChanges(WorkingCopy.java:201)
        at org.netbeans.api.java.source.JavaSource.runModificationTask(JavaSource.java:594)
        at org.netbeans.modules.refactoring.java.plugins.ExtractSuperclassRefactoringPlugin$CreateSuperclassElement.performChange(ExtractSuperclassRefactoringPlugin.java:351)
        at org.netbeans.modules.refactoring.api.RefactoringSession.doRefactoring(RefactoringSession.java:86)
        at org.netbeans.modules.refactoring.spi.impl.RefactoringPanel$1.run(RefactoringPanel.java:450)
        at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:539)
        at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:964)