OpenXML Filter: numbering defenition reading crashes the filtering

java.lang.NullPointerException
	at net.sf.okapi.filters.openxml.NumberingLevel$Default.id(NumberingLevel.java:78)
	at net.sf.okapi.filters.openxml.NumberingDefinition$Abstract.readWith(NumberingDefinition.java:114)
	at net.sf.okapi.filters.openxml.NumberingDefinitions$Default.readWith(NumberingDefinitions.java:136)
	at net.sf.okapi.filters.openxml.WordDocument.initialiseNumberingDefinitions(WordDocument.java:164)
	at net.sf.okapi.filters.openxml.WordDocument.open(WordDocument.java:68)
	at net.sf.okapi.filters.openxml.Document$General.open(Document.java:151)
	at net.sf.okapi.filters.openxml.OpenXMLFilter.openDocument(OpenXMLFilter.java:434)
	at net.sf.okapi.filters.openxml.OpenXMLFilter.next(OpenXMLFilter.java:258)

The above error happens when a document is built with the help of Apache POI. The numbering level definition in the word/numbering.xml part is not aligned with the spec and misses the required w:ilvl attribute.

Below is the UI of the attached example document:

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