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

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java.lang.NullPointerException:
at org.jgrapht.util.UnorderedVertexPair.hashCode(UnorderedVertexPair.java:71)
at java.util.HashMap.hash(HashMap.java:338)
at java.util.HashMap.get(HashMap.java:556)
at org.jgrapht.graph.specifics.FastLookupUndirectedSpecifics.getEdge(FastLookupUndirectedSpecifics.java:91)
at org.jgrapht.graph.AbstractBaseGraph.getEdge(AbstractBaseGraph.java:159)
at org.jgrapht.alg.HopcroftKarpBipartiteMatching.symmetricDifference(HopcroftKarpBipartiteMatching.java:182)
at org.jgrapht.alg.HopcroftKarpBipartiteMatching.maxMatching(HopcroftKarpBipartiteMatching.java:158)
at org.jgrapht.alg.HopcroftKarpBipartiteMatching.(HopcroftKarpBipartiteMatching.java:84)

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