Problemas com p:tree checkbox selection (primefaces)

Pessoal, boa tarde!
To com o seguinte problema: tenho uma pagina hxtml com JSF e Primefaces, usando 1 input, 1 componente p:tree com checkbox e 2 botoes. Quando nao seleciono nenhum checkbox e clico no botao salvar, nao da erro algum, porem quando marco e desmarco algum dos checkbox e clico num dos botões, aparece o seguinte erro anexo abaixo. Sera que alguem poderia me dar uma ajuda pra resolver esse problema? Agradeço desde já a atenção de vcs.

<h:form prependId="false" id="formCadastroPerfil">
				<p:message for="formCadastroPerfil" id="msgFormulario"/>
				<br/>
			    <div align="center">  
					<h:panelGrid columns="3">
		            	<h:outputLabel value="#{msgs['label.perfil']}" for="nomePerfil" style="font-size:12px;" /> 
		            	<p:inputText id="nomePerfil" maxlength="40" value="#{MBean.perfil.nomePerfil}" 
		            				 required="true" requiredMessage="#{msgs['campo.obrigatorio']}"/>
						<p:message for="nomePerfil" /> 
					</h:panelGrid>
					<br/>
<p:tree value="#{MBean.root}" var="node" id="checkboxTree" 
		selectionMode="checkbox"  selection="#{MBean.selectedNodes}" >  
		
	<p:treeNode>  
		<h:outputText value="#{node}" />  
	</p:treeNode>  
</p:tree>


 <p:commandButton value="#{msgs['button.cancelar']}"  action="//paginas/listaPerfis.jsf?faces-redirect=true" ajax="false" immediate="true" onclick="voltar();"/>

 <p:commandButton value="#{msgs['button.salvar']}" ajax="false" image="ui-icon-transfer-e-w" action="#{MBean.gravar}" update="checkboxTree" />
                 
</div>
				
			</h:form>

erro no console do eclipse:
java.lang.NullPointerException
	at org.primefaces.component.api.UITree.findTreeNode(UITree.java:104)
	at org.primefaces.component.api.UITree.setRowKey(UITree.java:73)
	at org.primefaces.component.tree.TreeRenderer.decodeSelection(TreeRenderer.java:67)
	at org.primefaces.component.tree.TreeRenderer.decode(TreeRenderer.java:36)
	at javax.faces.component.UIComponentBase.decode(UIComponentBase.java:787)
	at org.primefaces.component.api.UITree.processDecodes(UITree.java:167)
	at org.primefaces.component.tree.Tree.processDecodes(Tree.java:373)
	at javax.faces.component.UIForm.processDecodes(UIForm.java:225)
	at javax.faces.component.UIComponentBase.processDecodes(UIComponentBase.java:1176)
	at org.primefaces.component.panel.Panel.processDecodes(Panel.java:282)
	at javax.faces.component.UIComponentBase.processDecodes(UIComponentBase.java:1176)
	at javax.faces.component.UIComponentBase.processDecodes(UIComponentBase.java:1176)
	at org.primefaces.component.layout.Layout.processDecodes(Layout.java:229)
	at javax.faces.component.UIComponentBase.processDecodes(UIComponentBase.java:1176)
	at javax.faces.component.UIComponentBase.processDecodes(UIComponentBase.java:1176)
	at javax.faces.component.UIViewRoot.processDecodes(UIViewRoot.java:933)
	at com.sun.faces.lifecycle.ApplyRequestValuesPhase.execute(ApplyRequestValuesPhase.java:78)
	at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
	at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
	at javax.faces.webapp.FacesServlet.service(FacesServlet.java:593)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248)
	at org.primefaces.webapp.filter.FileUploadFilter.doFilter(FileUploadFilter.java:79)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:280)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248)
	at org.jboss.weld.servlet.ConversationPropagationFilter.doFilter(ConversationPropagationFilter.java:62)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:280)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:368)
	at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:109)
	at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:83)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:97)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:100)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:78)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:54)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:35)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.authentication.www.BasicAuthenticationFilter.doFilter(BasicAuthenticationFilter.java:177)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:187)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:105)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:79)
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
	at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:169)
	at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:237)
	at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:167)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:280)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161)
	at org.jboss.as.jpa.interceptor.WebNonTxEmCloserValve.invoke(WebNonTxEmCloserValve.java:50)
	at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877)
	at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:671)
	at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:930)
	at java.lang.Thread.run(Thread.java:722)