<!-- If you need to report a security issue please visit https://github.com/apache/dubbo/security/policy -->

- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 3.0.5
* Operating System version: xxx
* Java version: xxx

### Steps to reproduce this issue

Frequent exceptions in unit testing: 

```
[INFO] [02/11/21 06:31:32:300 UTC] ERROR listener.ServiceInstancesChangedListener:  [DUBBO] Failed to load service metadata, meta type is local, dubbo version: 3.0.5-SNAPSHOT, current host: 172.28.16.1
[INFO] java.lang.IllegalStateException: There's no ApplicationConfig specified.
[INFO] 	at org.apache.dubbo.config.context.ConfigManager.lambda$getApplicationOrElseThrow$0(ConfigManager.java:85)
[INFO] 	at java.util.Optional.orElseThrow(Optional.java:290)
[INFO] 	at org.apache.dubbo.config.context.ConfigManager.getApplicationOrElseThrow(ConfigManager.java:85)
[INFO] 	at org.apache.dubbo.rpc.model.ApplicationModel.getCurrentConfig(ApplicationModel.java:315)
[INFO] 	at org.apache.dubbo.registry.client.metadata.StandardMetadataServiceURLBuilder.generateUrlWithoutMetadata(StandardMetadataServiceURLBuilder.java:112)
[INFO] 	at org.apache.dubbo.registry.client.metadata.StandardMetadataServiceURLBuilder.build(StandardMetadataServiceURLBuilder.java:82)
[INFO] 	at org.apache.dubbo.registry.client.metadata.MetadataUtils.referProxy(MetadataUtils.java:93)
[INFO] 	at org.apache.dubbo.registry.client.metadata.MetadataUtils.lambda$getMetadataServiceProxy$0(MetadataUtils.java:67)
[INFO] 	at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1660)
[INFO] 	at org.apache.dubbo.registry.client.metadata.MetadataUtils.getMetadataServiceProxy(MetadataUtils.java:67)
[INFO] 	at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener.doGetMetadataInfo(ServiceInstancesChangedListener.java:379)
[INFO] 	at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener.getRemoteMetadata(ServiceInstancesChangedListener.java:330)
[INFO] 	at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener.onEvent(ServiceInstancesChangedListener.java:143)
[INFO] 	at org.apache.dubbo.registry.client.event.listener.ServiceInstancesChangedListener$AddressRefreshRetryTask.run(ServiceInstancesChangedListener.java:493)
[INFO] 	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
[INFO] 	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
[INFO] 	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
[INFO] 	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
[INFO] 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
[INFO] 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
[INFO] 	at java.lang.Thread.run(Thread.java:748)
```

import org.apache.dubbo.common.URLBuilder;
import org.apache.dubbo.common.config.ConfigurationUtils;
import org.apache.dubbo.common.logger.Logger;
import org.apache.dubbo.common.logger.LoggerFactory;
import org.apache.dubbo.metadata.MetadataService;
import org.apache.dubbo.registry.client.ServiceInstance;
import org.apache.dubbo.remoting.Constants;
import org.apache.dubbo.rpc.model.ApplicationModel;
import org.apache.dubbo.rpc.model.ScopeModelAware;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import static org.apache.dubbo.common.constants.CommonConstants.CONSUMER;
import static org.apache.dubbo.common.constants.CommonConstants.DUBBO_PROTOCOL;
import static org.apache.dubbo.common.constants.CommonConstants.GROUP_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.PORT_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.PROTOCOL_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.SIDE_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.TIMEOUT_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.VERSION_KEY;
import static org.apache.dubbo.metadata.MetadataConstants.DEFAULT_METADATA_TIMEOUT_VALUE;
import static org.apache.dubbo.metadata.MetadataConstants.METADATA_PROXY_TIMEOUT_KEY;
import static org.apache.dubbo.registry.client.metadata.ServiceInstanceMetadataUtils.getMetadataServiceURLsParams;
/**
 * Standard Dubbo provider enabling introspection service discovery mode.
 *
 * @see MetadataService
 * @since 2.7.5
 */
public class StandardMetadataServiceURLBuilder implements MetadataServiceURLBuilder, ScopeModelAware {