Listar dados com Hibernate - obtenho apenas NullPointerException

Bom dia, pessoal. Virei essa madrugada tentando implementar meu sistema. Já aprendi a gravar no banco de dados com o hibernate. Estou agora tentando fazer a listagem dos dados, mas só consigo a NullPointerException.

Essa e minha classe Bean:

public class Usuarios  implements java.io.Serializable {


     private Integer id;
     private String nome;
     private String email;
     private String senha;
     private String perfil;

// getters e setters
}

o metodo listar da classe DAO:

  public List<Usuarios> listarUsuario(){
             return this.session.createCriteria(Usuarios.class).list();
         }

Os metodos da lista na classe Controller:

public DataModel getListaUsuarios() {
        UsuarioDAOImpl ud = new UsuarioDAOImpl();
        listaUsuarios = new ListDataModel(ud.listarUsuario());
        return listaUsuarios;
    }

    public void setListaUsuarios(DataModel listaUsuarios) {
        this.listaUsuarios = listaUsuarios;
    }

E por fim, a pagina html:

 <ui:decorate template="painel.xhtml">
        <ui:define name="centro">
            <h:form>
                <p:dataTable value="#{usuarioController.listaUsuarios}" var="lista">
                    <p:column headerText="ID">
                        <p:outputLabel value="#{lista.id}"/>
                    </p:column>

Talvez seja o sono, mas nao to conseguindo ver porque estou pegando NUllPointerException. Poderiam dar uma força, por favor?

Erick, aqui está o stackTrace:

java.lang.NullPointerException
    at dao.UsuarioDAOImpl.listarUsuario(UsuarioDAOImpl.java:122)
    at controller.UsuarioController.getListaUsuarios(UsuarioController.java:37)
    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 javax.el.BeanELResolver.getValue(BeanELResolver.java:363)
    at com.sun.faces.el.DemuxCompositeELResolver._getValue(DemuxCompositeELResolver.java:176)
    at com.sun.faces.el.DemuxCompositeELResolver.getValue(DemuxCompositeELResolver.java:203)
    at com.sun.el.parser.AstValue.getValue(AstValue.java:138)
    at com.sun.el.parser.AstValue.getValue(AstValue.java:183)
    at com.sun.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:224)
    at org.jboss.weld.el.WeldValueExpression.getValue(WeldValueExpression.java:50)
    at com.sun.faces.facelets.el.TagValueExpression.getValue(TagValueExpression.java:109)
    at javax.faces.component.ComponentStateHelper.eval(ComponentStateHelper.java:194)
    at javax.faces.component.ComponentStateHelper.eval(ComponentStateHelper.java:182)
    at javax.faces.component.UIData.getValue(UIData.java:731)
    at org.primefaces.component.datatable.DataTable.getValue(DataTable.java:867)
    at org.primefaces.component.api.UIData.getDataModel(UIData.java:579)
    at javax.faces.component.UIData.getRowCount(UIData.java:356)
    at org.primefaces.component.datatable.DataTableRenderer.encodeTbody(DataTableRenderer.java:629)
    at org.primefaces.component.datatable.DataTableRenderer.encodeRegularTable(DataTableRenderer.java:234)
    at org.primefaces.component.datatable.DataTableRenderer.encodeMarkup(DataTableRenderer.java:196)
    at org.primefaces.component.datatable.DataTableRenderer.encodeEnd(DataTableRenderer.java:82)
    at javax.faces.component.UIComponentBase.encodeEnd(UIComponentBase.java:875)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1764)
    at javax.faces.render.Renderer.encodeChildren(Renderer.java:168)
    at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:845)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1757)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1760)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1760)
    at com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:402)
    at com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:131)
    at javax.faces.application.ViewHandlerWrapper.renderView(ViewHandlerWrapper.java:288)
    at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:121)
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
    at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:594)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1550)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:161)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:331)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
    at com.sun.enterprise.v3.services.impl.ContainerMapper$AdapterCallable.call(ContainerMapper.java:317)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:195)
    at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:860)
    at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:757)
    at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1056)
    at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:229)
    at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
    at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
    at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
    at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
    at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
    at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
    at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
    at java.lang.Thread.run(Thread.java:744)