getDio method
获取 Dio 实例
如果 key 不存在,抛出 StateError
Implementation
Dio getDio(String key) {
final dio = _pool[key];
if (dio == null) {
throw StateError(
'SmartClientManager: Client "$key" not found. '
'Please call register("$key", config) first.',
);
}
return dio;
}