querySelector is not working properly

[Livinglist]

The code snippet below is supposed to scrap all the sentences showed on https://jisho.org/search/%E6%97%A5%20%23sentences; however, the sentences after the execution of the function does not contain proper results.

For example, the selector #secondary > div > ul > li:nth-child(2) > div.sentence_content > ul > li:nth-child(1) > span.unlinked should return the element <span class="unlinked">今日</span> instead, it retuens null

List<Sentence> parseHtml() async {
    List<Sentence> sentences = [];
    Client client = Client();
    Response response = await client.get('https://jisho.org/search/%E6%97%A5%20%23sentences');
    var doc = parse(response.body);
    
    int childIndex;
    for (int i = 1; i <= 20; i++) {
      childIndex = 1;
      List<Token> tokens = [];
      while (true) {
        var element =
            doc.querySelector('#secondary > div > ul > li:nth-child($i) > div.sentence_content > ul > li:nth-child($childIndex) > span.unlinked');
        //print(element.text);
        if (element == null) break;
        var text = element.text;
        print("the i is $i and the currentIndex is $childIndex and the text is $text");
        element =
            doc.querySelector('#secondary > div > ul > li:nth-child($i) > div.sentence_content > ul > li:nth-child($childIndex) > span.furigana');
        if (element == null) {
          tokens.add(Token(text: text));
        } else {
          var furigana = element.text;
          tokens.add(Token(text: text, furigana: furigana));
        }
        childIndex++;
      }
        var sentence = Sentence(tokens: tokens);
        print(sentence.text);
        this.sentences.add(sentence);
        sentences.add(Sentence());
    }
    return sentences;
  }

class Token {
  String text;
  String furigana;

  Token({@required this.text, this.furigana});
}

class Sentence {
  List<Token> tokens;
  String text = '';

  Sentence({this.tokens}) {
    for (var token in tokens) {
      text += token.text;
    }
  }
}

[chen-zeyu]

"li:nth-child(2)" is not yet supported , #1018

[Livinglist]

Duplicate of #84