java.lang.NullPointerException
    at org.openscience.cdk.graph.SpanningTree.buildSpanningTree(SpanningTree.java:132)
    at org.openscience.cdk.graph.SpanningTree.<init>(SpanningTree.java:78)
    at org.openscience.cdk.ringsearch.AllRingsFinder.findAllRings(AllRingsFinder.java:131)
    at org.openscience.cdk.ringsearch.AllRingsFinder.findAllRings(AllRingsFinder.java:117)
    at de.ipbhalle.metfrag.fragmenter.Fragmenter.preprocessMolecule(Fragmenter.java:440)
    at de.ipbhalle.metfrag.fragmenter.Fragmenter.generateFragmentsInMemory(Fragmenter.java:668)
    at de.ipbhalle.metfrag.fragmenter.FragmenterThread.run(FragmenterThread.java:159)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:619)