空指针异常，托管Bean，新手求教

Question

public void changeType(ValueChangeEvent e) { FacesContext ctx = FacesContext.getCurrentInstance(); UIViewRoot root = ctx.getViewRoot(); HtmlSelectOneListbox subtypeListBox =  (HtmlSelectOneListbox)root.findComponent("booksubtype"); List<UIComponent> list =  subtypeListBox.getChildren();//此处为第33行 int count = list.size();以下为异常内容20:42:40,376 严重    [javax.enterprise.resource.webcontainer.jsf.context] (http--0.0.0.0-8080-1) /NewFile.xhtml @50,41 valueChangeListener="#{bookManager.changeType}": java.lang.NullPointerException: javax.faces.event.AbortProcessingException: /NewFile.xhtml @50,41 valueChangeListener="#{bookManager.changeType}": java.lang.NullPointerException at javax.faces.event.MethodExpressionValueChangeListener.processValueChange(MethodExpressionValueChangeListener.java:157) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final] at javax.faces.event.ValueChangeEvent.processListener(ValueChangeEvent.java:134) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final] at javax.faces.component.UIComponentBase.broadcast(UIComponentBase.java:769) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final] at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:759) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final] at javax.faces.component.UIViewRoot.processDecodes(UIViewRoot.java:935) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final] at com.sun.faces.lifecycle.ApplyRequestValuesPhase.execute(ApplyRequestValuesPhase.java:78) [jsf-impl-2.1.7-jbossorg-2.jar:] at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101) [jsf-impl-2.1.7-jbossorg-2.jar:] at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118) [jsf-impl-2.1.7-jbossorg-2.jar:] at javax.faces.webapp.FacesServlet.service(FacesServlet.java:593) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329) [jbossweb-7.0.13.Final.jar:] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [jbossweb-7.0.13.Final.jar:] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161) [jbossweb-7.0.13.Final.jar:] at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153) [jboss-as-web-7.1.1.Final.jar:7.1.1.Final] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155) [jbossweb-7.0.13.Final.jar:] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [jbossweb-7.0.13.Final.jar:] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [jbossweb-7.0.13.Final.jar:] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368) [jbossweb-7.0.13.Final.jar:] at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [jbossweb-7.0.13.Final.jar:] at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:671) [jbossweb-7.0.13.Final.jar:] at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:930) [jbossweb-7.0.13.Final.jar:] at java.lang.Thread.run(Thread.java:744) [rt.jar:1.7.0_45]Caused by: java.lang.NullPointerException at bookManager.changeType(bookManager.java:33) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.7.0_45] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) [rt.jar:1.7.0_45] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.7.0_45] at java.lang.reflect.Method.invoke(Method.java:606) [rt.jar:1.7.0_45] at org.apache.el.parser.AstValue.invoke(AstValue.java:262) [jbossweb-7.0.13.Final.jar:] at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:278) [jbossweb-7.0.13.Final.jar:] at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:105) [jsf-impl-2.1.7-jbossorg-2.jar:] at javax.faces.event.MethodExpressionValueChangeListener.processValueChange(MethodExpressionValueChangeListener.java:144) [jboss-jsf-api_2.1_spec-2.0.1.Final.jar:2.0.1.Final] ... 21 more