please check whether related SPI module is missing

- [ ] I have searched the [issues](https://github.com/apache/dubbo-admin/issues) of this repository and believe that this is not a duplicate.
- [x ] I have checked the [REAMDE](https://github.com/apache/dubbo-admin/blob/develop/README.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo Admin version: 0.4.0
* Registry center detail (type and version):
* Operating System version:7.5
* Java version: 1.8

### Steps to reproduce this issue

Pls. provide [GitHub address] to reproduce this issue.

https://github.com/apache/dubbo-admin.git

What actually happens?

If there is an exception, please attach the exception trace:

java.lang.IllegalStateException: No such extension org.apache.dubbo.registry.client.ServiceDiscovery by name dubbo, no related exception was found, please check whether related SPI module is missing.
at org.apache.dubbo.common.extension.ExtensionLoader.findException(ExtensionLoader.java:688)
at org.apache.dubbo.common.extension.ExtensionLoader.createExtension(ExtensionLoader.java:695)
at org.apache.dubbo.common.extension.ExtensionLoader.getExtension(ExtensionLoader.java:495)
at org.apache.dubbo.common.extension.ExtensionLoader.getExtension(ExtensionLoader.java:475)
at org.apache.dubbo.registry.client.DefaultServiceDiscoveryFactory.createDiscovery(DefaultServiceDiscoveryFactory.java:46)
at org.apache.dubbo.registry.client.AbstractServiceDiscoveryFactory.lambda$getServiceDiscovery$0(AbstractServiceDiscoveryFactory.java:45)
at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1660)
at org.apache.dubbo.registry.client.AbstractServiceDiscoveryFactory.getServiceDiscovery(AbstractServiceDiscoveryFactory.java:45)
at org.apache.dubbo.registry.client.ServiceDiscoveryRegistry.getServiceDiscovery(ServiceDiscoveryRegistry.java:141)
at org.apache.dubbo.registry.client.ServiceDiscoveryRegistry.createServiceDiscovery(ServiceDiscoveryRegistry.java:119)
at org.apache.dubbo.registry.client.ServiceDiscoveryRegistry.(ServiceDiscoveryRegistry.java:104)
at org.apache.dubbo.registry.client.ServiceDiscoveryRegistryFactory.createRegistry(ServiceDiscoveryRegistryFactory.java:35)
at org.apache.dubbo.registry.support.AbstractRegistryFactory.getRegistry(AbstractRegistryFactory.java:159)
at org.apache.dubbo.registry.RegistryFactoryWrapper.getRegistry(RegistryFactoryWrapper.java:34)
at org.apache.dubbo.registry.RegistryFactory$Adaptive.getRegistry(RegistryFactory$Adaptive.java)
at org.apache.dubbo.registry.integration.RegistryProtocol.getRegistry(RegistryProtocol.java:359)
at org.apache.dubbo.registry.integration.RegistryProtocol.export(RegistryProtocol.java:214)
at org.apache.dubbo.rpc.protocol.ProtocolListenerWrapper.export(ProtocolListenerWrapper.java:64)
Just put your stack trace here!
```

Start by reproducing the reported exception from the dubbo-admin repository with Java 1.8, then trace ExtensionLoader.java through DefaultServiceDiscoveryFactory.java and the ServiceDiscovery registry classes named in the stack trace. Verify whether the SPI module providing ServiceDiscovery named "dubbo" is present; done means the missing-module cause is identified and the issue has a clear resolution.

Written by the indexing model from the issue text.