运行dubbo-samples-zookeeper，提示No such extension org.apache.dubbo.rpc.Protocol by name dubbo

Exception in thread "main" java.lang.IllegalStateException: No such extension org.apache.dubbo.rpc.Protocol by name dubbo
at org.apache.dubbo.common.extension.ExtensionLoader.findException(ExtensionLoader.java:618)
at org.apache.dubbo.common.extension.ExtensionLoader.createExtension(ExtensionLoader.java:625)
at org.apache.dubbo.common.extension.ExtensionLoader.getExtension(ExtensionLoader.java:424)
at org.apache.dubbo.config.ServiceConfig.findConfigedPorts(ServiceConfig.java:644)
at org.apache.dubbo.config.ServiceConfig.doExportUrlsFor1Protocol(ServiceConfig.java:444)
at org.apache.dubbo.config.ServiceConfig.doExportUrls(ServiceConfig.java:325)
at org.apache.dubbo.config.ServiceConfig.doExport(ServiceConfig.java:300)
at org.apache.dubbo.config.ServiceConfig.export(ServiceConfig.java:206)
at org.apache.dubbo.config.bootstrap.DubboBootstrap.lambda$exportServices$16(DubboBootstrap.java:943)
at java.util.HashMap$Values.forEach(HashMap.java:981)
at org.apache.dubbo.config.bootstrap.DubboBootstrap.exportServices(DubboBootstrap.java:930)
at org.apache.dubbo.config.bootstrap.DubboBootstrap.start(DubboBootstrap.java:749)
at org.apache.dubbo.config.spring.context.DubboBootstrapApplicationListener.onContextRefreshedEvent(DubboBootstrapApplicationListener.java:59)
at org.apache.dubbo.config.spring.context.DubboBootstrapApplicationListener.onApplicationContextEvent(DubboBootstrapApplicationListener.java:52)
at org.apache.dubbo.config.spring.context.OneTimeExecutionApplicationContextEventListener.onApplicationEvent(OneTimeExecutionApplicationContextEventListener.java:40)
at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:393)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:347)
at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:883)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:546)
at org.springframework.context.annotation.AnnotationConfigApplicationContext.(AnnotationConfigApplicationContext.java:84)
at org.apache.dubbo.samples.ProviderBootstrap.main(ProviderBootstrap.java:33)

Reproduce the failure by running the dubbo-samples-zookeeper sample and start at ProviderBootstrap.java:33, where startup reaches the reported exception. Trace the protocol lookup through ExtensionLoader.java and ServiceConfig.java, then verify that the sample starts without the missing dubbo Protocol error.

Written by the indexing model from the issue text.