Flutter 发生异常。SocketException（SocketException:主机查找失败：'；字符串'；（操作系统错误：没有与主机名关联的地址，errno=7））

我正在用flatter编写一个模块，它返回指定URL的预览。但是它返回以下错误？如何解决这个问题

我得到的错误是：

发生异常。 SocketException（SocketException:主机查找失败：“字符串”（操作系统错误：没有与主机名关联的地址，Errno=7））

我尝试了以下方法

代码：

import 'package:html/parser.dart';
import 'package:http/http.dart';

class FetchPreview {
  Future fetch(url) async {
    final client = Client();
    final response = await client.get(_validateUrl(url));
    final document = parse(response.body);

    String description, title, image, appleIcon, favIcon;

    var elements = document.getElementsByTagName('meta');
    final linkElements = document.getElementsByTagName('link');

    elements.forEach((tmp) {
      if (tmp.attributes['property'] == 'og:title') {
        //fetch seo title
        title = tmp.attributes['content'];
      }
      //if seo title is empty then fetch normal title
      if (title == null || title.isEmpty) {
        title = document.getElementsByTagName('title')[0].text;
      }

      //fetch seo description
      if (tmp.attributes['property'] == 'og:description') {
        description = tmp.attributes['content'];
      }
      //if seo description is empty then fetch normal description.
      if (description == null || description.isEmpty) {
        //fetch base title
        if (tmp.attributes['name'] == 'description') {
          description = tmp.attributes['content'];
        }
      }

      //fetch image
      if (tmp.attributes['property'] == 'og:image') {
        image = tmp.attributes['content'];
      }
    });

    linkElements.forEach((tmp) {
      if (tmp.attributes['rel'] == 'apple-touch-icon') {
        appleIcon = tmp.attributes['href'];
      }
      if (tmp.attributes['rel']?.contains('icon') == true) {
        favIcon = tmp.attributes['href'];
      }
    });

    return {
      'title': title ?? '',
      'description': description ?? '',
      'image': image ?? '',
      'appleIcon': appleIcon ?? '',
      'favIcon': favIcon ?? ''
    };
  }

  _validateUrl(String url) {
    if (url?.startsWith('http://') == true ||
        url?.startsWith('https://') == true) {
      return url;
    } else {
      return 'http://$url';
    }
  }
}