How to send Data to Xpath in Flutter

Question

I have an app which listens serial port. When data has came, i need to set textbox value in web view. Text box has no id, no name, i need to set value with xpath. But when data came from serial port, i can't set the textbox value. I am not sure, what is the problem.

      _serialData = "";
    
        if (_subscription != null) {
          _subscription!.cancel();
          _subscription = null;
        }
    
        if (_transaction != null) {
          _transaction!.dispose();
          _transaction = null;
        }
    
        if (_port != null) {
          _port!.close();
          _port = null;
        }
    
        if (device == null) {
          _device = null;
          setState(() {
            _status = "Bağlantı Koptu!";
          });
          return true;
        }
    
        _port = await device.create();
        if (await (_port!.open()) != true) {
          setState(() {
            _status = "Bağlantı Noktası Açılamadı";
          });
          return false;
        }
        _device = device;
    
        await _port!.setDTR(true);
        await _port!.setRTS(true);
        await _port!.setPortParameters(
            1000000, UsbPort.DATABITS_8, UsbPort.STOPBITS_1, UsbPort.PARITY_NONE);
    
        _transaction = Transaction.stringTerminated(
            _port!.inputStream as Stream<Uint8List>, Uint8List.fromList([13, 10]));
    
        _subscription = _transaction!.stream.listen((String line) {
          setState(() {
            if (_serialData != line) {
              _serialData = line;
              webViewController!
                  .runJavascript(
                  "var aranan = document.evaluate('/html/body/div/div/div[2]/div/div[2]/div[2]/div/div/form/div/input',document,null,XPathResult.FIRST_ORDERED_NODE_TYPE,null).singleNodeValue;
aranan.value='$line';"
              );
            }
          });
        });

Problem is _transaction!.stream.listen((String line) function. I can get data from serial port. There is no problem there. But i can't set this xpath "/html/body/div/div/div[2]/div/div[2]/div[2]/div/div/form/div/input"