提交表单信息时老是报这个错，请问是怎么回事啊？
Struts Problem Report
Struts has detected an unhandled exception: 

Messages: attempt to create saveOrUpdate event with null entity 
 
File: org/hibernate/event/SaveOrUpdateEvent.java 
Line number: 40 


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Stacktraces
java.lang.IllegalArgumentException: attempt to create saveOrUpdate event with null entity 
  org.hibernate.event.SaveOrUpdateEvent.(SaveOrUpdateEvent.java:40)
  org.hibernate.event.SaveOrUpdateEvent.(SaveOrUpdateEvent.java:23)
  org.hibernate.impl.SessionImpl.save(SessionImpl.java:523)
  org.hibernate.impl.SessionImpl.save(SessionImpl.java:519)
  org.springframework.orm.hibernate3.HibernateTemplate$12.doInHibernate(HibernateTemplate.java:693)
  org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:419)
  org.springframework.orm.hibernate3.HibernateTemplate.executeWithNativeSession(HibernateTemplate.java:374)
  org.springframework.orm.hibernate3.HibernateTemplate.save(HibernateTemplate.java:690)
  com.exercise.hibernate.impl.SalChanceDaoImpl.saveSalChance(SalChanceDaoImpl.java:22)
  com.exercise.service.impl.SalChanceServiceImpl.saveSalChance(SalChanceServiceImpl.java:23)
  sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
  sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
  java.lang.reflect.Method.invoke(Unknown Source)
  org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:310)
  org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182)
  org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149)
  org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:106)
  org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
  org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89)
  org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
  org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
  $Proxy0.saveSalChance(Unknown Source)
  com.exercise.struts.SalChanceAction.addSalChance(SalChanceAction.java:23)
  sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
  sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
  java.lang.reflect.Method.invoke(Unknown Source)
  com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:441)
  com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:280)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:243)
  com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:165)
  com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)
  com.opensymphony.xwork2.validator.ValidationIntercept