javax.des visages.el.EvaluationException: java.lang.NullPointerException dans mon Application web jsf

Je suis en train de travailler sur une Application Web dynamique et, comme première étape, je veux utiliser mon jsf page pour enregistrer un nouveau Groupe dans la Base de données .Je suis de la connexion à une Base de données Oracle 10g, travail sur eclipse Kepler, avec Glassfish4 et à l'aide de primefaces 4.0.Ce la StackTrace

                  2013-12-07T00:38:34.028+0100|Avertissement: #{groupeBean.createGroupe}: java.lang.NullPointerException
javax.faces.FacesException: #{groupeBean.createGroupe}: java.lang.NullPointerException
at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:118)
at javax.faces.component.UICommand.broadcast(UICommand.java:315)
at javax.faces.component.UIData.broadcast(UIData.java:1108)
at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:790)
at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:1282)
at com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:81)
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:198)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:646)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:357)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:55)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
at java.lang.Thread.run(Thread.java:724)
Caused by: javax.faces.el.EvaluationException: java.lang.NullPointerException
at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:101)
at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
... 35 more
Caused by: java.lang.NullPointerException
at com.portail.dao.DaoGroupe.ajouter(DaoGroupe.java:21)
at com.portail.managedBeans.GroupeBean.createGroupe(GroupeBean.java:24)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.sun.el.parser.AstValue.invoke(AstValue.java:275)
at com.sun.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:304)
at org.jboss.weld.util.el.ForwardingMethodExpression.invoke(ForwardingMethodExpression.java:40)
at org.jboss.weld.el.WeldMethodExpression.invoke(WeldMethodExpression.java:50)
at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:105)
at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:87)
... 36 more

C'est le code de la managedBean

                @ManagedBean
@ViewScoped
public class GroupeBean {
private Groupe newGroupe=new Groupe();
public Groupe getNewGroupe() {
return newGroupe;
}
public void setNewGroupe(Groupe newGroupe) {
this.newGroupe = newGroupe;
}
private DaoGroupe gdao= new DaoGroupe();
public void createGroupe()
{
gdao.ajouter(newGroupe);
}}

C'est le code de la Classe Dao

            public class DaoGroupe {
private static final String JPA_UNIT_NAME="Portail";
private EntityManager entityManager;
protected EntityManager getEntityManager() {
if (entityManager == null) {
entityManager = Persistence.createEntityManagerFactory(
JPA_UNIT_NAME).createEntityManager();
}
return entityManager;
}
public   void ajouter(Groupe g)
{
EntityTransaction tx = entityManager.getTransaction();
tx.begin();
entityManager.persist(g);
tx.commit();}}

Et c'est l'onglet insertion, dans mon jsf page

                  <p:tab title="Groupe">
<h:form>
<h:panelGrid columns="2">
<h:outputText value="Nom : *" />
<p:inputText value="#{groupeBean.newGroupe.nom}" required="true"
label="nomgrp" validatorMessage="Nom de Groupe Obligatoire" />
<h:outputText value="Numéro : *" />
<p:inputText value="#{groupeBean.newGroupe.idGroupe}" required="true"
label="numgrp" validatorMessage="Numéro de Groupe Obligatoire" />
</h:panelGrid>
<p:commandButton value="Ajouter"
style="width:205px;margin-left:10%"
action="#{groupeBean.createGroupe}" />
</h:form>
</p:tab>