java.lang.IllegalArgumentException: node to traverse cannot be null!

java.lang.IllegalArgumentException: node to traverse cannot be null!
	org.hibernate.hql.ast.util.NodeTraverser.traverseDepthFirst(NodeTraverser.java:31)
	org.hibernate.hql.ast.QueryTranslatorImpl.parse(QueryTranslatorImpl.java:254)
	org.hibernate.hql.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:157)
	org.hibernate.hql.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:111)
	org.hibernate.engine.query.HQLQueryPlan.<init>(HQLQueryPlan.java:77)
	org.hibernate.engine.query.HQLQueryPlan.<init>(HQLQueryPlan.java:56)
	org.hibernate.engine.query.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:72)
	org.hibernate.impl.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:133)
	org.hibernate.impl.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:112)
	org.hibernate.impl.SessionImpl.createQuery(SessionImpl.java:1623)
	org.springframework.orm.hibernate3.HibernateTemplate$30.doInHibernate(HibernateTemplate.java:919)
	org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:419)
	org.springframework.orm.hibernate3.HibernateTemplate.executeWithNativeSession(HibernateTemplate.java:374)
	org.springframework.orm.hibernate3.HibernateTemplate.find(HibernateTemplate.java:917)
	org.springframework.orm.hibernate3.HibernateTemplate.find(HibernateTemplate.java:909)
	com.xinlong.dao.impl.BaseDaoImpl.findEntity(BaseDaoImpl.java:51)
	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)
	$Proxy7.findEntity(Unknown Source)
	com.xinlong.xinlongmis.service.impl.WkDocTypeService.findGetAllWkDocType(WkDocTypeService.java:59)
	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)
	$Proxy15.findGetAllWkDocType(Unknown Source)
	com.xinlong.xinlongmis.front.action.WkDocAction.GetDoctypebyId(WkDocAction.java:161)
	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:440)
	com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:279)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
	com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:163)
	com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:249)
	org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
	com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)
	com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)
	com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:148)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:93)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:235)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:128)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	org.apache.struts2.interceptor.ProfilingActivationInterceptor.intercept(ProfilingActivationInterceptor.java:104)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138)
	com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:148)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:128)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176)
	com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
	org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)
	org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:468)
	org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:395)
	org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:96)
	org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)

问题解决:

出现这种问题是因为HQL语句出现问题，引起内部查询对象为空，无法处理为空值引起的。解决方法，检查HQL语句（尤其是关键字）。