javax.el.PropertyNotFoundException：无法到达目标，标识符[NumLikesBean]解析为null

如何解决javax.el.PropertyNotFoundException：无法到达目标，标识符[NumLikesBean]解析为null

需要在我的jsf网络应用中更新值。我进行了更多探索，找不到适合我的结果。 XHTML代码：

<p:column>  
   <f:facet name="header">NumLikes</f:facet>  
   <h:outputText id="numLikes_id" value="#{NumLikesBean.numLikes}" /> 
   <p:poll interval="3" listener="#{NumLikesBean.process}" update="form1:numLikes_id" />
</p:column>

Bean类

package User;
import java.io.Serializable;
import javax.faces.bean.ViewScoped;
//@Named
@ViewScoped
public class NumLikesBean implements Serializable  {
    private int numLikes;
    public int getNumLikes() {
        return numLikes;
    }
    public void process() {
        numLikes++;  
    }
}

每3秒，在Tomcat中收到此错误消息。

28-Aug-2020 11:41:15.231 WARNING [http-nio-8082-exec-4] com.sun.faces.lifecycle.InvokeApplicationPhase.execute /index.xhtml @68,100 listener="#{NumLikesBean.process}": Target Unreachable,identifier [NumLikesBean] resolved to null
javax.el.PropertyNotFoundException: /index.xhtml @68,identifier [NumLikesBean] resolved to null
    at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:107)
    at org.primefaces.component.poll.Poll.broadcast(Poll.java:51)
    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.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)at java.base/java.lang.Thread.run(Thread.java:830)