我在Spring WebFlow中使用JSF。我在使用PrimeFaces时遇到以下异常，但它适用于PrimeFaces 3.4。
HTTP Status 500 - Request processing failed; nested exception is org.springframework.webflow.execution.FlowExecutionException: Exception thrown in state 'signUp' of flow 'main'
type Exception report
message Request processing failed; nested exception is org.springframework.webflow.execution.FlowExecutionException: Exception thrown in state 'signUp' of flow 'main'
description The server encountered an internal error that prevented it from fulfilling this request.
exception
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.webflow.execution.FlowExecutionException: Exception thrown in state 'signUp' of flow 'main'
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:894)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
root cause
org.springframework.webflow.execution.FlowExecutionException: Exception thrown in state 'signUp' of flow 'main'
org.springframework.webflow.engine.impl.FlowExecutionImpl.wrap(FlowExecutionImpl.java:571)
org.springframework.webflow.engine.impl.FlowExecutionImpl.resume(FlowExecutionImpl.java:262)
org.springframework.webflow.executor.FlowExecutorImpl.resumeExecution(FlowExecutorImpl.java:169)
org.springframework.webflow.mvc.servlet.FlowHandlerAdapter.handle(FlowHandlerAdapter.java:183)
org.springframework.faces.webflow.JsfFlowHandlerAdapter.handle(JsfFlowHandlerAdapter.java:48)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
root cause
javax.faces.FacesException
com.sun.faces.context.ExceptionHandlerImpl.handle(ExceptionHandlerImpl.java:142)
com.sun.faces.lifecycle.Phase.doPhase(Phase.java:119)
com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
org.springframework.faces.webflow.FlowLifecycle.render(FlowLifecycle.java:80)
org.springframework.faces.webflow.JsfView.render(JsfView.java:89)
org.springframework.webflow.engine.ViewState.render(ViewState.java:296)
org.springframework.webflow.engine.ViewState.refresh(ViewState.java:243)
org.springframework.webflow.engine.ViewState.resume(ViewState.java:221)
org.springframework.webflow.engine.Flow.resume(Flow.java:545)
org.springframework.webflow.engine.impl.FlowExecutionImpl.resume(FlowExecutionImpl.java:258)
org.springframework.webflow.executor.FlowExecutorImpl.resumeExecution(FlowExecutorImpl.java:169)
org.springframework.webflow.mvc.servlet.FlowHandlerAdapter.handle(FlowHandlerAdapter.java:183)
org.springframework.faces.webflow.JsfFlowHandlerAdapter.handle(JsfFlowHandlerAdapter.java:48)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
root cause
java.lang.NullPointerException
org.primefaces.renderkit.CoreRenderer.getWidgetBuilder(CoreRenderer.java:481)
org.primefaces.component.ajaxstatus.AjaxStatusRenderer.encodeScript(AjaxStatusRenderer.java:41)
org.primefaces.component.ajaxstatus.AjaxStatusRenderer.encodeEnd(AjaxStatusRenderer.java:34)
javax.faces.component.UIComponentBase.encodeEnd(UIComponentBase.java:875)
javax.faces.component.UIComponent.encodeAll(UIComponent.java:1786)
javax.faces.component.UIComponent.encodeAll(UIComponent.java:1782)
javax.faces.component.UIComponent.encodeAll(UIComponent.java:1782)
com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:424)
com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:125)
org.springframework.faces.webflow.FlowViewHandler.renderView(FlowViewHandler.java:99)
com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:121)
com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:139)
org.springframework.faces.webflow.FlowLifecycle.render(FlowLifecycle.java:80)
org.springframework.faces.webflow.JsfView.render(JsfView.java:89)
org.springframework.webflow.engine.ViewState.render(ViewState.java:296)
org.springframework.webflow.engine.ViewState.refresh(ViewState.java:243)
org.springframework.webflow.engine.ViewState.resume(ViewState.java:221)
org.springframework.webflow.engine.Flow.resume(Flow.java:545)
org.springframework.webflow.engine.impl.FlowExecutionImpl.resume(FlowExecutionImpl.java:258)
org.springframework.webflow.executor.FlowExecutorImpl.resumeExecution(FlowExecutorImpl.java:169)
org.springframework.webflow.mvc.servlet.FlowHandlerAdapter.handle(FlowHandlerAdapter.java:183)
org.springframework.faces.webflow.JsfFlowHandlerAdapter.handle(JsfFlowHandlerAdapter.java:48)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
note The full stack trace of the root cause is available in the Apache Tomcat/7.0.42 logs.
这是如何引起的？如何解决？