runtime error: invalid memory address or nil pointer dereference

``` func UpdateAgent(agentInfo *model.AgentUpdateInfo) { sql := "" if g.Config().Hosts == "" { sql = fmt.Sprintf( "insert into host(hostname, ip, agent_version, plugin_version) values ('%s', '%s', '%s', '%s') on duplicate key update ip='%s', agent_version='%s', plugin_version='%s'", agentInfo.ReportRequest.Hostname, agentInfo.ReportRequest.IP, agentInfo.ReportRequest.AgentVersion, agentInfo.ReportRequest.PluginVersion, agentInfo.ReportRequest.IP, agentInfo.ReportRequest.AgentVersion, agentInfo.ReportRequest.PluginVersion, ) } else { // sync, just update sql = fmt.Sprintf( "update host set ip='%s', agent_version='%s', plugin_version='%s' where hostname='%s'", agentInfo.ReportRequest.IP, agentInfo.ReportRequest.AgentVersion, agentInfo.ReportRequest.PluginVersion, agentInfo.ReportRequest.Hostname, ) } defer func() { if err := recover(); err != nil { log.Panicln("出错了:", err) } }() _, err := DB.Exec(sql) if err != nil { log.Println("exec", sql, "fail", err) } else { url := g.Config().GroupHostAddUrl fmt.Println(url) var hosts, group_name string hosts = agentInfo.ReportRequest.Hostname group_name = g.Config().DefaultHostGroup var jsonStr = []byte(`{"hosts":"` + hosts + `","group_name":"` + group_name + `"}`) req, err := http.NewRequest("POST", url, bytes.NewBuffer(jsonStr)) req.Header.Set("Content-Type", "application/json") client := &http.Client{} resp, err := client.Do(req) defer resp.Body.Close() if err != nil { log.Println("Post To Portal API Fail:", sql, err) } } } ``` 报指针为空错误错： ``` panic: 出错了: runtime error: invalid memory address or nil pointer dereference ``` 新手求解