java.lang.NullPointerException: This exception has no message.

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java.lang.NullPointerException:
at org.semanticweb.elk.util.collections.ArrayHashMap.get(ArrayHashMap.java:203)
at org.semanticweb.elk.util.collections.chains.AbstractChain$1.next(AbstractChain.java:105)
at org.semanticweb.elk.util.collections.chains.AbstractChain$1.next(AbstractChain.java:101)
at org.semanticweb.elk.util.collections.chains.AbstractChain.find(AbstractChain.java:42)
at org.semanticweb.elk.reasoner.indexing.hierarchy.IndexedSubClassOfAxiom$ThisCompositionRule.removeFrom(IndexedSubClassOfAxiom.java:162)
at org.semanticweb.elk.reasoner.indexing.hierarchy.DifferentialIndex.add(DifferentialIndex.java:172)
at org.semanticweb.elk.reasoner.indexing.hierarchy.IndexedSubClassOfAxiom.updateOccurrenceNumbers(IndexedSubClassOfAxiom.java:85)
at org.semanticweb.elk.reasoner.indexing.hierarchy.MainAxiomIndexerVisitor$AxiomOccurrenceUpdateFilter.update(MainAxiomIndexerVisitor.java:384)
at org.semanticweb.elk.reasoner.indexing.hierarchy.MainAxiomIndexerVisitor$AxiomOccurrenceUpdateFilter.visit(MainAxiomIndexerVisitor.java:394)
at org.semanticweb.elk.reasoner.indexing.hierarchy.MainAxiomIndexerVisitor$AxiomOccurrenceUpdateFilter.visit(MainAxiomIndexerVisitor.java:373)
at org.semanticweb.elk.reasoner.indexing.hierarchy.MainAxiomIndexerVisitor.indexSubClassOfAxiom(MainAxiomIndexerVisitor.java:148)
at org.semanticweb.elk.reasoner.indexing.hierarchy.AbstractElkAxiomIndexerVisitor.visit(AbstractElkAxiomIndexerVisitor.java:166)
at org.semanticweb.elk.reasoner.indexing.hierarchy.AbstractElkAxiomIndexerVisitor.visit(AbstractElkAxiomIndexerVisitor.java:63)
at org.semanticweb.elk.owlapi.wrapper.ElkEquivalentClassesAxiomWrap.accept(ElkEquivalentClassesAxiomWrap.java:61)
at org.semanticweb.elk.owlapi.wrapper.ElkClassAxiomWrap.accept(ElkClassAxiomWrap.java:51)
at org.semanticweb.elk.reasoner.indexing.hierarchy.DelegatingElkAxiomVisitor.visit(DelegatingElkAxiomVisitor.java:103)
at org.semanticweb.elk.reasoner.indexing.hierarchy.DelegatingElkAxiomVisitor.visit(DelegatingElkAxiomVisitor.java:74)
at org.semanticweb.elk.owlapi.wrapper.ElkEquivalentClassesAxiomWrap.accept(ElkEquivalentClassesAxiomWrap.java:61)
at org.semanticweb.elk.owlapi.wrapper.ElkClassAxiomWrap.accept(ElkClassAxiomWrap.java:51)
at org.semanticweb.elk.reasoner.indexing.hierarchy.ChangeIndexingProcessor.visit(ChangeIndexingProcessor.java:57)
at org.semanticweb.elk.owlapi.OwlOntologyChangeProcessorVisitor.visit(OwlOntologyChangeProcessorVisitor.java:78)
at org.semanticweb.elk.owlapi.OwlOntologyChangeProcessorVisitor.visit(OwlOntologyChangeProcessorVisitor.java:39)
at org.semanticweb.owlapi.model.AddAxiom.accept(AddAxiom.java:76)
at org.semanticweb.elk.owlapi.OwlChangesLoader.load(OwlChangesLoader.java:90)
at org.semanticweb.elk.reasoner.stages.AxiomLoadingStage.executeStage(AxiomLoadingStage.java:121)
at org.semanticweb.elk.reasoner.stages.AbstractReasonerStage.execute(AbstractReasonerStage.java:165)
at org.semanticweb.elk.reasoner.stages.AxiomLoadingStage.execute(AxiomLoadingStage.java:45)
at org.semanticweb.elk.reasoner.stages.LoggingStageExecutor.execute(LoggingStageExecutor.java:53)
at org.semanticweb.elk.reasoner.stages.AbstractStageExecutor.complete(AbstractStageExecutor.java:55)
at org.semanticweb.elk.reasoner.stages.AbstractReasonerState.complete(AbstractReasonerState.java:145)
at org.semanticweb.elk.reasoner.stages.AbstractReasonerState.loadAxioms(AbstractReasonerState.java:306)
at org.semanticweb.elk.reasoner.stages.AbstractReasonerState.isInconsistent(AbstractReasonerState.java:284)
at org.semanticweb.elk.reasoner.stages.AbstractReasonerState.getTaxonomy(AbstractReasonerState.java:324)
at org.semanticweb.elk.owlapi.ElkReasoner.precomputeInferences(ElkReasoner.java:942)
at simplification.ConceptMinimizer.computeConjunctions(ConceptMinimizer.java:254)

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