title: NullPointerException in drools 5.5.0-final when retracting
body: <p>I´ve got a problem with the following rule:</p>
  
  <pre><code>rule "Término sin Traducción"
      salience -100
      dialect "mvel"
      when
          traductor : TraductorDeEventosTratados()
              eventoGenerico : EventoGenerico() from traductor.eventoGenerico
      then
          System.out.println("Evento generico: " + eventoGenerico);
              traductor.setEventoGenerico( null );
              update( traductor );
              retract( eventoGenerico );
  end
  </code></pre>
  
  <p>It causes a <code>NullPointerException</code> when retracting the "eventoGenerico", as though it doesn't exist in the working memory (it exists actually, and another rule sets the <code>eventoGenerico</code> to <code>traductor</code> previously):</p>
  
  <pre><code>Exception executing consequence for rule "Término sin Traducción" in RULA_PROV.SYSTEM_RULES: [Error: drools.retract( eventoGenerico ): null]
  [Near : {... System.out.println("Evento gen ....}]
               ^
  [Line: 1, Column: 1]
          at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleException(DefaultConsequenceExceptionHandler.java:39)
          at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1297)
  </code></pre>
  
  <p>However, if I make this little change it works fine (this verifies that the eventoGenerico really exists in the working memory):</p>
  
  <pre><code>rule "Término sin Traducción"
      salience -100
      dialect "mvel"
      when
          traductor : TraductorDeEventosTratados()
              eventoGenerico : EventoGenerico()
              eventoGenerico2 : EventoGenerico( this == eventoGenerico ) from traductor.eventoGenerico
      then
          System.out.println("Evento generico: " + eventoGenerico);
              traductor.setEventoGenerico( null );
              update( traductor );
              retract( eventoGenerico );
  end
  </code></pre>
  
  <p>It looks like a bug, any ideas?</p>
  
  <p>Thanks in advance</p>