XML parsing error in Dart

Im having an error in my Dart app with XML parsing, here is the code segment for the parsing:

getValue(value, string) {
    var substring = string.substr(string.indexOf(value));
    return substring.substr(value.length + 2, substring.indexOf('"', value.length + 2) - (value.length + 2));
}

The error I get is :

Unhandled exception:
Uncaught Error: The null object does not have a method 'indexOf'.

NoSuchMethodError: method not found: 'indexOf'
Receiver: null
Arguments: ["cb"]
Stack Trace:
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:45)
#1      getValue (file:///C:/Users/karim_000/dart/test/bin/test.dart:5:49)
#2      connectChat (file:///C:/Users/karim_000/dart/test/bin/test.dart:31:35)
#3      loginUser.<anonymous closure> (file:///C:/Users/karim_000/dart/test/bin/test.dart:19:18)
#4      _RootZone.runUnary (dart:async/zone.dart:1151)
#5      _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:496)
#6      _Future._propagateToListeners (dart:async/future_impl.dart:579)
#7      _Future._completeWithValue (dart:async/future_impl.dart:339)
#8      _Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:401)
#9      _asyncRunCallbackLoop (dart:async/schedule_microtask.dart:41)
#10     _asyncRunCallback (dart:async/schedule_microtask.dart:48)
#11     _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:84)
#12     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:131)

#0      _rootHandleUncaughtError.<anonymous closure> (dart:async/zone.dart:883)
#1      _asyncRunCallbackLoop (dart:async/schedule_microtask.dart:41)
#2      _asyncRunCallback (dart:async/schedule_microtask.dart:48)
#3      _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:84)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:131)

here is the whole code:

import 'dart:io';
import 'dart:core';

getValue(value, string) {
    var substring = string.substr(string.indexOf(value));
    return substring.substr(value.length + 2, substring.indexOf('"', value.length + 2) - (value.length + 2));
}

loginUser(id, username, pass) {
  var data;  
  var logindata;
    Socket.connect('173.255.132.116', 10014).then((socket) {
      print("connected to logen server");  
      socket.write('<y r="8" v="0" u="' + id + '" />\0');
      socket.write('<v p="' + pass + '" n="' + username + '" />\0'); 
      print("Sending Authentication information");
      socket.close();
        print("Disconnected from login server");
      connectChat(logindata, '211258656', '882763542', 'VenomshqB0t', 'ratscake');


    });
  return logindata;
}
connectChat(login, chatid, userid, username, password) {

    var data = true;

    var y = '<y r="' + chatid + '" m="1" v="0" u="' + userid + '" />\0';

    var j2 = '<j2 cb="' + getValue("cb", login) + '" q="1" y="' + getValue("i", y) + '" k="' + getValue("k1", login) + '" k3="' + getValue("k3", login) + '" p="0" c="' + chatid + '" r="0" f="0" u="' + userid + '" d0="' + getValue("d0", login) + '" d3="' + getValue("d3", login) + '" dt="' + getValue("dt", login) + '" N="' + username + '" n="BotDART" a="http://ift.tt/1uSPjl0" h="" v="0" />\0';

    print("Trying to connect");

    var socket = Socket.connect('173.255.132.116', 10010).then((socket) {
         print("Authenticating user");
         socket.write(y);
         socket.write(j2);
    });
}
void main() {
  loginUser('882763542', 'venomshqb0t', 'ratscake');
}

Any help or advise? Thank you!