我fork的main分支，这个地方panic，看看是否需要修复

[alan2016llg]

Environment

• Server:
• Client:
• Protocol:
• Registry:

Issue description

Logs

Click me to check logs

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x1cd2871]

goroutine 7179829 [running]:
dubbo.apache.org/dubbo-go/v3/registry/servicediscovery.(*ServiceInstancesChangedListenerImpl).OnEvent(0xc0044bb940, {0x2d07e10?, 0xc00f77e960})
        /home/zmn/go/pkg/mod/github.com/alan2016llg/dubbo-go/[email protected]/registry/servicediscovery/service_instances_changed_listener_impl.go:125 +0x10d1
dubbo.apache.org/dubbo-go/v3/registry/zookeeper.(*zookeeperServiceDiscovery).DataChange(0xc000397180, {{0xc006fa2f60, 0x2b}, 0x0, {0xc007b47180, 0x32a}})
        /home/zmn/go/pkg/mod/github.com/alan2016llg/dubbo-go/[email protected]/registry/zookeeper/service_discovery.go:282 +0x557
dubbo.apache.org/dubbo-go/v3/remoting/zookeeper.(*ZkEventListener).handleZkNodeEvent(0xc003732570, {0xc010254018, 0x17}, {0xc00b14c010, 0x1, 0x1?}, {0x2ce3ce0, 0xc000397180})
        /home/zmn/go/pkg/mod/github.com/alan2016llg/dubbo-go/[email protected]/remoting/zookeeper/listener.go:212 +0x536
dubbo.apache.org/dubbo-go/v3/remoting/zookeeper.(*ZkEventListener).startScheduleWatchTask(0xc003732570, {0xc001d2de78, 0x17}, {0xc00b14c010, 0x1, 0x1}, 0x8bb2c97000, {0x2ce3ce0, 0xc000397180}, 0xc00415b380)
        /home/zmn/go/pkg/mod/github.com/alan2016llg/dubbo-go/[email protected]/remoting/zookeeper/listener.go:454 +0x5ae
dubbo.apache.org/dubbo-go/v3/remoting/zookeeper.(*ZkEventListener).listenDirEvent(0xc003732570, 0x0, {0xc001d2de78, 0x17}, {0x2ce3ce0?, 0xc000397180}, {0x0, 0x0})
        /home/zmn/go/pkg/mod/github.com/alan2016llg/dubbo-go/[email protected]/remoting/zookeeper/listener.go:422 +0x5b1
dubbo.apache.org/dubbo-go/v3/remoting/zookeeper.(*ZkEventListener).ListenServiceEvent.func1({0xc001d2de78, 0x17}, {0x2ce3ce0?, 0xc000397180?})
        /home/zmn/go/pkg/mod/github.com/alan2016llg/dubbo-go/[email protected]/remoting/zookeeper/listener.go:480 +0xb7
created by dubbo.apache.org/dubbo-go/v3/remoting/zookeeper.(*ZkEventListener).ListenServiceEvent
        /home/zmn/go/pkg/mod/github.com/alan2016llg/dubbo-go/[email protected]/remoting/zookeeper/listener.go:475 +0x1be

[FoghostCn]

pls provide more information such as config file or init logic

[alan2016llg]

@FoghostCn 我这边是泛化调用，配置的是双注册方式，配置如下：
`nacosregistryConfig := &config.RegistryConfig{
Protocol: "nacos",
Address: "localhost:8848,",
Timeout: "10s",
Username: "nacos",
Password: "nacos",
Namespace: "dev_dubbo",
Preferred: true,
}
zkegistryConfig := &config.RegistryConfig{
Protocol: "zookeeper",
Address: "localhost:2181",
Timeout: "10s",
Preferred: false,
}

method := config.MethodConfig{InterfaceName: iface, Retries: "0"}
methods := []*config.MethodConfig{&method}
refConf := config.ReferenceConfig{
	InterfaceName:  iface,
	Cluster:        "failover",
	RegistryIDs:    []string{"nacos", "zk"},
	Protocol:       protocol,
	Generic:        "true",
	Methods:        methods,
	Version:        "1.0",
	RequestTimeout: "9s",
	ProvidedBy:     "example-dubbo",
}

rootConfig := config.NewRootConfigBuilder().
	AddRegistry("nacos", nacosregistryConfig).
	AddRegistry("zk", zkegistryConfig).
	Build()
if err := config.Load(config.WithRootConfig(rootConfig)); err != nil {
	panic(err)
}
_ = refConf.Init(rootConfig)
refConf.GenericLoad("example-dubbo")`

[alan2016llg]

@FoghostCn

[alan2016llg]

@FoghostCn 我大概知道为什么没有获取到了、在日志中发现了如下err日志： ERROR servicediscovery/service_instances_changed_listener_impl.go:256 get metadata of 192.168.50.52 failed, java exception:Server side(192.168.50.52,10324) thread pool is exhausted, detail msg:Task org.apache.dubbo.remoting.transport.dispatcher.ChannelEventRunnable@60590860 rejected from java.util.concurrent.ThreadPoolExecutor@479ca085[Running, pool size = 100, active threads = 100, queued tasks = 0, completed tasks = 53] ，在这种情况下，会导致没获取到，然后上层方法panic了