JDO java.lang.NullPointerException

Question

I am very new to GAE, JDO and JAVA itself and got myself into a NullPointerException while posting my form data to the Servlet. The above code is my JSP file with the Form:

  <form action="/todolist" method="POST">
  <div class="form-group">
    <label for="titel">Titel</label>
    <input type="text" class="form-control" id="titel" name="titel"   placeholder="Titel">
  </div>
  <div class="form-group">
    <label for="beschreibung">Beschreibung</label>
    <input type="text" class="form-control" id="beschreibung" name="beschreibung" placeholder="Beschreibung">
  </div>
  <div class="form-group">
  <label for="duration">Dauer</label>
  <input type="text" class="form-control" id="duration" name="duration" placeholder="zB. 2 Wochen...">
</div>
  <div class="form-group">
  <label for="prio">Priorität</label>
<select class="form-control" id="prio" name="prio">
<option value="1">Priorität 1</option>
<option value="2">Priorität 2</option>
<option value="3">Priorität 3</option>
<option value="4">Priorität "P"</option>
</select>
  </div>

    </div>
  <div class="modal-footer">
  <button type="submit" class="btn btn-primary" name="save">Save changes</button>
    <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>    
    </form>
  </div>
</div>

This is the Servlet which receives the submitted Fields:

public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    String beschreibung = req.getParameter("beschreibung");
    String prio = req.getParameter("prio");
    String titel = req.getParameter("titel");
    String duration = req.getParameter("duration");
    Date termin = new Date();


    req.setAttribute("beschreibung", beschreibung);
    req.setAttribute("prio", prio);
    req.setAttribute("titel", titel);
    req.setAttribute("duration", duration);



    PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("transactions-optional");
    PersistenceManager pm = pmf.getPersistenceManager();


    ToDo doing = new ToDo(titel, beschreibung, termin, duration, prio, "super");


      try {
            pm.makePersistent(doing);
        } finally {
            pm.close();
        }


    RequestDispatcher rd = req.getRequestDispatcher("../WEB-INF/index.jsp");
    rd.forward(req, resp);

}

UPDATE: I have initialized all my values and am still getting the NullPointerException