Qianyi951015 opened a new issue, #21816:
URL: https://github.com/apache/shardingsphere/issues/21816
## Bug Report
### Which version of ShardingSphere did you use?
62005f74f508ff0e5ae3d5882fbbf0cd19c4784f
### Which project did you use? ShardingSphere-JDBC or ShardingSphere-Proxy?
Proxy
### Expected behavior
db_discovery type dropped
### Actual behavior
when execute `drop db_discovery rule` exception cannot found db_discovery
type
### Steps to reproduce the behavior, such as: SQL to execute, sharding rule
configuration, when exception occur etc.
1. Register storage units
```sql
register storage unit
ds_0(url="jdbc:mysql://xxx.xxx.xxx.xxx:3306/primary_demo_ds?serverTimezone=UTC&useSSL=false",
user="root",password="xxxxx"),
ds_1(url="jdbc:mysql://xxx.xxx.xxx.xxx:3307/primary_demo_ds?serverTimezone=UTC&useSSL=false",
user="root",password="xxxxx"),
ds_2(url="jdbc:mysql://xxx.xxx.xxx.xxx:3308/primary_demo_ds?serverTimezone=UTC&useSSL=false",
user="root",password="xxxxx");
```
2. Create db_discovery rule
```sql
CREATE DB_DISCOVERY RULE group_0 ( STORAGE_UNITS(ds_0, ds_1, ds_2),
TYPE(NAME='MySQL.MGR',PROPERTIES('group-name'='666edd3c-02ec-11ea-9bb3-000027e33bd2')),
HEARTBEAT(PROPERTIES('keep-alive-cron'='0/5 * * * * ?')) );
```
3. Drop db_discovery type
```sql
DROP DB_DISCOVERY TYPE group_0_MySQL.MGR FROM test0;
```
### Screen shot
<img width="749" alt="image"
src="https://user-images.githubusercontent.com/57847965/198498682-d107a50e-0ae7-46d4-a5d1-2dd9076cf8e8.png";>
### Log
```
[ERROR] 2022-10-28 11:27:30.135 [Curator-SafeNotifyService-0]
c.g.common.eventbus.EventBus.default - Exception thrown by subscriber method
renew(org.apache.shardingsphere.mode.manager.cluster.coordinator.registry.status.storage.event.StorageNodeChangedEvent)
on subscriber
org.apache.shardingsphere.mode.manager.cluster.coordinator.ClusterContextManagerCoordinator@79b84841
when dispatching event:
org.apache.shardingsphere.mode.manager.cluster.coordinator.registry.status.storage.event.StorageNodeChangedEvent@4a15eb17
java.lang.NullPointerException: Can not find database discovery data source
rule in database `test0`
```
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