Minor Description
For non-grouped searches QueryComponent.regularFinishStage already considers the possibility of null SolrDocument values due to an index change.
For grouped searches GroupedEndResultTransformer.transform potentially adds a null element to a SolrDocumentList.
The TextResponseWriter.writeSolrDocumentList method passes any null SolrDocument through to the JSONWriter.writeSolrDocument method leading to a NullPointerException at line 87.