XLSX2CSV.main() has thrown a NullPointerException

We found this prefix in 3 webpages.
MapChildren (3)Typical messages (1)TraceDetailed Map
  1. java.lang.NullPointerException
    1.   at excel_test.XSSFReader$SheetIterator.hasNext()
    2.   at excel_test.XLSX2CSV.process()
    3.   at excel_test.XLSX2CSV.main()
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MessageNumber of crashes
<No message>3
  1. java.lang.NullPointerException
    1.   at excel_test.XSSFReader$SheetIterator.hasNext()
    2.   at excel_test.XLSX2CSV.process()
    3.   at excel_test.XLSX2CSV.main()