The method '+' was called on null

My main.dart file is below. When i run the program, _totalScore = _totalScore + score; part gives the error of
‘Exception has occurred.
NoSuchMethodError (NoSuchMethodError: The method ‘+’ was called on null.
Receiver: null
Tried calling: +(3))’

Can someone help me?

import ‘package:firstguide/result.dart’;
import ‘package:flutter/material.dart’;

import ‘./quiz.dart’;
import ‘./result.dart’;

void main() {
runApp(MyApp()); //it calls the widget
}

class MyApp extends StatefulWidget {
//it was a stateless widget we cahnged to stateful widget

@override
State createState() {
return _MyAppState();
}
}

class _MyAppState extends State {
//stateful widget
final _questions = const [
//using mapping (key value pairs)
{
‘questionText’: ‘Whats your favorite color?’,
‘answers’: [
{‘text’: ‘Black’, ‘score’: 10},
{‘text’: ‘Red’, ‘score’: 5},
{‘text’: ‘Green’, ‘score’: 3},
{‘text’: ‘White’, ‘score’: 1},
]
},
{
‘questionText’: ‘What’s your favorite animal?’,
‘answers’: [
{‘text’: ‘Cat’, ‘score’: 5},
{‘text’: ‘Dog’, ‘score’: 3},
{‘text’: ‘Horse’, ‘score’: 0},
{‘text’: ‘Lion’, ‘score’: 9},
]
},

{
  'questionText': 'Who\'s your favorite instructor?',
  'answers': [
    {'text': 'Sel', 'score': 10},
    {'text': 'Sel', 'score': 10},
    {'text': 'Sel', 'score': 10},
    {'text': 'Sel', 'score': 10},
  ]
},

];

var _questionIndex = 0;
var _totalScore;

void _answerQuestion(int score) {
_totalScore = _totalScore + score;

setState(() {
  _questionIndex = _questionIndex + 1;
});

print((_questionIndex));

if (_questionIndex < _questions.length) {
  print('We have more questions!');
} else {
  print('No more questions.');
}

}

@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text(‘My first app’),
),
body: _questionIndex < _questions.length
? Quiz(
answerQuestion: _answerQuestion,
questionIndex: _questionIndex,
questions: _questions,
)
: Result(_totalScore),
),
);
}
}