LUCENE-5023 it was reported that composite reader contexts should not contain null fields() readers. But this is wrong, as a null-fields() reader may contain documents, just no fields.
fields() and terms() is documented to return null, so DocSets should check for null (like all queries do in Lucene). It seems that DocSetNC does not correctly check for null.
- is broken by
LUCENE-5023 Only reader that contains fields can be added into readerContext