double check that there are mappings before trying to call the post process method

modules/elasticsearch/src/main/java/org/elasticsearch/index/mapper/xcontent/XContentDocumentMapper.java

@@ -521,12 +521,14 @@ public class XContentDocumentMapper implements DocumentMapper, ToXContent {
     @Override public void processDocumentAfterIndex(Document doc) {
         for (Fieldable field : doc.getFields()) {
             FieldMappers fieldMappers = mappers().indexName(field.name());
+            if (fieldMappers != null) {
                 FieldMapper mapper = fieldMappers.mapper();
                 if (mapper != null) {
                     mapper.processFieldAfterIndex(field);
                 }
             }
         }
+    }
 
     @Override public synchronized MergeResult merge(DocumentMapper mergeWith, MergeFlags mergeFlags) {
         XContentDocumentMapper xContentMergeWith = (XContentDocumentMapper) mergeWith;