title: solrj Query with child-parent relationsship fails
body: <p>I have the following solrj query:</p>
  
  <pre><code>    HttpSolrClient solr = new HttpSolrClient.Builder("http://10.xx.xxx.xxx:8983/solr").build();
      solr.setParser(new XMLResponseParser());
      SolrQuery query = new SolrQuery();
      query.set("q", "*:*");
      query.setFilterQueries("rt_tag:[2017-01-20T00:00:00Z TO NOW]","_relationship_:parent");
      query.setFields("[child parentFilter=\"_relationship_:parent\"]","rt_*");       
      QueryResponse response = solr.query("rtm_aggregations_shard1_replica1", query);
      SolrDocumentList docList = response.getResults();
  </code></pre>
  
  <p>When i execute this query, I get the following error:</p>
  
  <pre><code>&gt; ERROR SolrDispatchFilter null:java.lang.NullPointerException
  &gt; null:java.lang.NullPointerException   at
  &gt; org.apache.solr.response.transform.ChildDocTransformer.transform(ChildDocTransformerFactory.java:137)
  &gt;   at
  &gt; org.apache.solr.response.BinaryResponseWriter$Resolver.writeResultsBody(BinaryResponseWriter.java:159)
  &gt;   at
  &gt; org.apache.solr.response.BinaryResponseWriter$Resolver.writeResults(BinaryResponseWriter.java:183)
  &gt;   at
  &gt; org.apache.solr.response.BinaryResponseWriter$Resolver.resolve(BinaryResponseWriter.java:88)
  &gt;   at
  &gt; org.apache.solr.common.util.JavaBinCodec.writeVal(JavaBinCodec.java:158)
  &gt;   at
  &gt; org.apache.solr.common.util.JavaBinCodec.writeNamedList(JavaBinCodec.java:148)
  &gt;   at
  &gt; org.apache.solr.common.util.JavaBinCodec.writeKnownType(JavaBinCodec.java:242)
  &gt;   at
  &gt; org.apache.solr.common.util.JavaBinCodec.writeVal(JavaBinCodec.java:153)
  &gt;   at
  &gt; org.apache.solr.common.util.JavaBinCodec.marshal(JavaBinCodec.java:96)
  </code></pre>
  
  <p>However, executing the corresponding query per http directly works fine:</p>
  
  <pre><code>http://10.xx.xxx.xxx:8983/solr/rtm_aggregations_shard1_replica1/select?q=*:*&amp;fq=rt_tag:[2017-01-20T00:00:00Z%20TO%20NOW]&amp;fq=_relationship_:parent&amp;fl=[child%20parentFilter=%22_relationship_:parent%22],rt_*
  </code></pre>
  
  <p>Why does it not work with solrj?</p>