Group by query reports null pointer when unique key is not stored

[rajani m]

Hi Solr Users,

  Group by query is failing with the following error message. It looks like
TopGroupsResultTransformer.java
<https://github.com/apache/solr/blob/main/solr/core/src/java/org/apache/solr/search/grouping/distributed/shardresultserializer/TopGroupsResultTransformer.java#L237C24-L237C40>
retrieveDocument method is fetching doc based on id is calling a stored
field visitor method. I tried setting the "id" field to use doc values as
they are enabled "useDocValuesAsStored:true" but this didn't help. Any
alternative?

Would you call this a bug or improvement?

Error message -

java.lang.NullPointerException: Cannot invoke
"org.apache.lucene.index.IndexableField.stringValue()" because "f" is
null => java.lang.NullPointerException: Cannot invoke
"org.apache.lucene.index.IndexableField.stringValue()" because "f" is
null
        at org.apache.solr.schema.FieldType.toExternal(FieldType.java:378)
java.lang.NullPointerException: Cannot invoke
"org.apache.lucene.index.IndexableField.stringValue()" because "f" is
null
        at org.apache.solr.schema.FieldType.toExternal(FieldType.java:378) 
~[?:?]
        at 
org.apache.solr.search.grouping.distributed.shardresultserializer.TopGroupsResultTransformer.serializeTopGroups(TopGroupsResultTransformer.java:238)
~[?:?]