title: solr join function to query documents in multiple cores NullPointerException
body: <p>I use solr join to query documents from two cores, my cores is defined as follows:
  Post core:</p>
  
  <pre><code>        &lt;fields&gt;   
        &lt;!-- general --&gt;
      &lt;field name="id"type="string"indexed="true"stored="true" multiValued="false" required="true"/&gt;
  
      &lt;field name="creatorId"type="string"indexed="true"stored="true"multiValued="false" required="true"/&gt;
      .
      .
      .
  
  &lt;/fields&gt;
  </code></pre>
  
  <p>User core:</p>
  
  <pre><code> &lt;fields&gt;   
    &lt;!-- general --&gt;
    &lt;field name="id"        type="string"    indexed="true"  stored="true"  multiValued="false" required="true"/&gt;
    &lt;field name="username"      type="string"    indexed="true"  stored="true"  multiValued="false" /&gt; 
    &lt;field name="email"      type="string"    indexed="true"  stored="true"  multiValued="false" /&gt; 
    &lt;field name="userBrief"      type="string"    indexed="true"  stored="true"  multiValued="false" /&gt; 
    &lt;field name="jobNumber"      type="string"    indexed="true"  stored="true"  multiValued="false" /&gt;   
  &lt;/fields&gt;
  </code></pre>
  
  <p>now I want to query all user who has created post, I use join function, my url is like this:</p>
  
  <pre><code>http://localhost:9080/solr/user/select?q=*:*&amp;fq={!join from=creatorId to=id fromIndex=post}
  </code></pre>
  
  <p>but it don't work, and it throw a exception:</p>
  
  <pre><code>null: java.lang.NullPointerException
        at org.apache.lucene.search.IndexSearcher.rewrite(IndexSearcher.java:559)
        at org.apache.lucene.search.IndexSearcher.createNormalizedWeight(IndexSearcher.java:646)
        at org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:280)
        .
        .
        .
  </code></pre>
  
  <p>I don't know why, can you help me?</p>