java.lang.IllegalStateException: ref not allow null!

java.lang.IllegalStateException: ref not allow null!
                                   at com.alibaba.dubbo.config.ServiceConfig.checkRef(ServiceConfig.java:252)
                                   at com.alibaba.dubbo.config.ServiceConfig.doExport(ServiceConfig.java:207)
                                   at com.alibaba.dubbo.config.ServiceConfig.export(ServiceConfig.java:145)
                                   at com.alibaba.dubbo.config.spring.ServiceBean.onApplicationEvent(ServiceBean.java:110)
                                   at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:97)
                                   at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:303)
                                   at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:911)
                                   at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:428)
                                   at org.springframework.web.context.ContextLoader.createWebApplicationContext(ContextLoader.java:276)
                                   at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:197)
                                   at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:47)
                                   at com.caucho.server.webapp.WebApp.startImpl(WebApp.java:3677)
                                   at com.caucho.server.webapp.WebApp$StartupTask.run(WebApp.java:5188)
                                   at com.caucho.env.thread2.ResinThread2.runTasks(ResinThread2.java:173)
                                   at com.caucho.env.thread2.ResinThread2.run(ResinThread2.java:118)