title: NullPointerException when use the PBCTypeDCurveGenerator of jPBC
body: <p>Recently, I need to use jPBC to implement a bilinear pairing. But when I Use the Type D Curves just as the <a href="http://gas.dia.unisa.it/projects/jpbc/docs/ecpg.html#TypeD" rel="nofollow">http://gas.dia.unisa.it/projects/jpbc/docs/ecpg.html#TypeD</a> says. I encountered the Null Pointer Exception. Can you help me out ？  Thanks a lot.</p>
  
  <p>Here is the code :</p>
  
  <pre><code>  public class JPBC {
      public static void main(String[] args) throws Exception{
          int discriminant = 9563;
          PBCTypeDCurveGenerator dg = new PBCTypeDCurveGenerator(discriminant);
          dg.generate();   //&lt;&lt;&lt;-------------goes wrong !
       }
  }
  </code></pre>
  
  <p>Here is the hint:</p>
  
  <pre><code>Exception in thread "main" java.lang.NullPointerException 
      at it.unisa.dia.gas.plaf.jpbc.pbc.curve.PBCTypeDCurveGenerator.pbcGenerate(PBCTypeDCurveGenerator.java:19)
      at it.unisa.dia.gas.plaf.jpbc.pbc.curve.PBCCurveGenerator.generate(PBCCurveGenerator.java:17)
      at JPBC.main(JPBC.java:10)
  </code></pre>