Error 500 jsp Help

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accept
 <label required>Departamentos</label>
  <select class="form-control" onchange="buscarProvincia(this);" id="nIdeDptoNac">                  
           <option value="${cmb.nIdeCompendio}">--Seleccione--</option>                            
           <c:forEach var="cmb" items="${combo}">
           <option value="${cmb.nIdeCompendio}">${cmb.vNomCorto}</option>
           </c:forEach>
                            
 </select>

I'm trying to fill a combobox (select) in a jsp with servlet and jstl but it sends me error to a line of code, I check it and everything is fine, if you can help me !!! ... I'll leave it to you I have so far The error sends me on line 34 of the servlet (null pointer) the mysql select returns data

public class ComboDAOimpl {
Connection conn = null;
PreparedStatement pstm = null;
ResultSet rs = null;
CatDetCompendioDTO catDetCompendioDTO =null;

public List<CatDetCompendioDTO> llenarCombos(String sql){
    List<CatDetCompendioDTO> listaCombo = new ArrayList<CatDetCompendioDTO>();
    try {
        conn = Conexion.getConexion();
        pstm = conn.prepareStatement(sql);
        rs = pstm.executeQuery();
        while(rs.next()) {
            catDetCompendioDTO = new CatDetCompendioDTO();
            catDetCompendioDTO.setnIdeDetalle(rs.getInt("nIdeDetalle"));
            catDetCompendioDTO.setvNomCorto(rs.getString("vNomCorto"));
            catDetCompendioDTO.setvNomLargo(rs.getString("vNomLargo"));
            listaCombo.add(catDetCompendioDTO);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

    return listaCombo;
}

----------------------------------------------------------

@WebServlet ("/ comboController") public class comboController extends HttpServlet {     private static final long serialVersionUID = 1L;

public comboController() {
    super();
    // TODO Auto-generated constructor stub
}
ComboService comboService;

protected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
    req.setAttribute("combo", comboService.listarDepartamentosService());
    req.getRequestDispatcher("listaPeliculas.jsp").forward(req, res);
}

public class ComboServiceImpl implements ComboService {

 private ComboDAO comboDepartamento;

public List<CatDetCompendioDTO> listarDepartamentosService() {

    List<CatDetCompendioDTO> listaDepartamento = new ArrayList<CatDetCompendioDTO>();
    StringBuffer sql = new StringBuffer();
    sql.append("select nIdeDetalle,vNomCorto,vNomLargo from catdetcompendio where nidecompendio=9  order by vNomCorto");
    try {
        listaDepartamento   = comboDepartamento.llenarCombos(sql.toString());
    }catch (Exception e) {
        e.printStackTrace();
    }
    return listaDepartamento;
}

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