fix NPE

wrapper_db_client.go

@@ -257,6 +257,9 @@ func (dc *DBClient) SetTableStructure(tableConfigTable map[string][]*define.Colu
 		dc.cacheTableStructureConfig.Enable = false
 	}
 	dc.lock.Lock()
+	if nil == dc.tableStructureCache {
+		dc.tableStructureCache = make(map[string][]*define.ColumnConfig)
+	}
 	for table, columnConfig := range tableConfigTable {
 		dc.tableStructureCache[table] = columnConfig
 	}