No provider available error of dubbo-samples-chain

dubbo-samples-chain sometimes reports an error: No provider available

org.apache.dubbo.samples.chain.api.TigerService has been registered, but it cannot be found in a later call.

```
[10/01/21 11:24:23:023 UTC] main INFO config.ServiceConfig: [DUBBO] Register dubbo service org.apache.dubbo.samples.chain.api.TigerService url dubbo://172.19.0.3:20880/org.apache.dubbo.samples.chain.api.TigerService?anyhost=true&application=chain-backend-app&bind.ip=172.19.0.3&bind.port=20880&deprecated=false&dubbo=2.0.2&dynamic=true&generic=false&interface=org.apache.dubbo.samples.chain.api.TigerService&methods=getName,getId,tiger&pid=16&release=2.7.7&revision=1.0-SNAPSHOT&side=provider×tamp=1610277863256 to registry registry://zookeeper:2181/org.apache.dubbo.registry.RegistryService?application=chain-backend-app&dubbo=2.0.2&pid=16®istry=zookeeper&release=2.7.7×tamp=1610277863240, dubbo version: 2.7.7, current host: 172.19.0.3
```

```
[10/01/21 11:24:36:036 UTC] DubboServerHandler-172.19.0.4:20881-thread-10 ERROR filter.ExceptionFilter: [DUBBO] Got unchecked and undeclared exception which called by 172.19.0.5. service: org.apache.dubbo.samples.chain.api.ChineseService, method: watch, exception: org.apache.dubbo.rpc.RpcException: No provider available from registry zookeeper:2181 for service org.apache.dubbo.samples.chain.api.TigerService on consumer 172.19.0.4 use dubbo version 2.7.7, please check status of providers(disabled, not registered or in blacklist)., dubbo version: 2.7.7, current host: 172.19.0.4
2021-01-10T11:24:49.7600283Z org.apache.dubbo.rpc.RpcException: No provider available from registry zookeeper:2181 for service org.apache.dubbo.samples.chain.api.TigerService on consumer 172.19.0.4 use dubbo version 2.7.7, please check status of providers(disabled, not registered or in blacklist).
```

actions log: https://github.com/apache/dubbo-samples/runs/1676491180?check_suite_focus=true

or download from here: [log file](https://github.com/apache/dubbo-samples/files/5793872/7_Run.tests.txt)