title: NullPointerException when generating PDF using Docx4j
body: <p>This is the error my app throws:</p>
  
  <pre><code>Caused by: java.lang.NullPointerException
      at org.docx4j.convert.out.fo.renderers.FORendererApacheFOP.render(FORendererApacheFOP.java:199)
      at org.docx4j.convert.out.fo.renderers.FORendererApacheFOP.render(FORendererApacheFOP.java:159)
      at org.docx4j.convert.out.fo.AbstractFOExporter.postprocess(AbstractFOExporter.java:168)
      at org.docx4j.convert.out.fo.AbstractFOExporter.postprocess(AbstractFOExporter.java:47)
      at org.docx4j.convert.out.common.AbstractExporter.export(AbstractExporter.java:82)
      ... 63 more
  </code></pre>
  
  <p><strong>Code:</strong></p>
  
  <pre><code>InputStream is = ByteSource.wrap(bytes).openStream();
  WordprocessingMLPackage wordMLPackage = WordprocessingMLPackage.load(is);
  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  FOSettings foSettings = Docx4J.createFOSettings();
  foSettings.setWmlPackage(wordMLPackage);
  Docx4J.toFO(foSettings, baos, Docx4J.FLAG_EXPORT_PREFER_XSL);
  </code></pre>
  
  <p>The line which throws NPE is </p>
  
  <pre><code>Docx4J.toFO(foSettings, baos, Docx4J.FLAG_EXPORT_PREFER_XSL);
  </code></pre>
  
  <p>What could be wrong here?</p>