Unityにてスコアランキングを実装しようとしています。

Unity初心者です。
スコアランキングを実装しようとしており、以下のサイトを参考にしました。
https://blog.naichilab.com/entry/webgl-simple-ranking

以下のエラーが出ているのですが、原因はわかりますでしょうか？
NullReferenceException: Object reference not set to an instance of an object
naichilab.RankingSceneManager.get_BoardIdPlayerPrefsKey () (at Assets/naichilab/unity-simple-ranking/Scripts/RankingSceneManager.cs:44)
naichilab.RankingSceneManager.Start () (at Assets/naichilab/unity-simple-ranking/Scripts/RankingSceneManager.cs:75)

C#
1using System.Collections;
2using UnityEngine;
3using UnityEngine.UI;
4using System.Linq;
5using NCMB;
6using NCMB.Extensions;
7
8namespace naichilab
9{
10    public class RankingSceneManager : MonoBehaviour
11    {
12        private const string OBJECT_ID = "objectId";
13        private const string COLUMN_SCORE = "score";
14        private const string COLUMN_NAME = "name";
15
16
17        [SerializeField] Text captionLabel;
18        [SerializeField] Text scoreLabel;
19        [SerializeField] Text highScoreLabel;
20        [SerializeField] InputField nameInputField;
21        [SerializeField] Button sendScoreButton;
22        [SerializeField] Button closeButton;
23        [SerializeField] RectTransform scrollViewContent;
24        [SerializeField] GameObject rankingNodePrefab;
25        [SerializeField] GameObject readingNodePrefab;
26        [SerializeField] GameObject notFoundNodePrefab;
27        [SerializeField] GameObject unavailableNodePrefab;
28
29        private string _objectid = null;
30
31        private string ObjectID
32        {
33            get { return _objectid ?? (_objectid = PlayerPrefs.GetString(BoardIdPlayerPrefsKey, null)); }
34            set
35            {
36                if (_objectid == value)
37                    return;
38                PlayerPrefs.SetString(BoardIdPlayerPrefsKey, _objectid = value);
39            }
40        }
41
42        private string BoardIdPlayerPrefsKey
43        {
44            get { return string.Format("{0}_{1}_{2}", "board", _board.ClassName, OBJECT_ID); }
45        }
46
47        private RankingInfo _board;
48        private IScore _lastScore;
49
50        private NCMBObject _ncmbRecord;
51
52        /// <summary>
53        /// 入力した名前
54        /// </summary>
55        /// <value>The name of the inputted.</value>
56        private string InputtedNameForSave
57        {
58            get
59            {
60                if (string.IsNullOrEmpty(nameInputField.text))
61                {
62                    return "名無し";
63                }
64
65                return nameInputField.text;
66            }
67        }
68
69        void Start()
70        {
71            sendScoreButton.interactable = false;
72            _board = RankingLoader.Instance.CurrentRanking;
73            _lastScore = RankingLoader.Instance.LastScore;
74
75            Debug.Log(BoardIdPlayerPrefsKey + "=" + PlayerPrefs.GetString(BoardIdPlayerPrefsKey, null));
76
77            StartCoroutine(GetHighScoreAndRankingBoard());
78        }
79
80        IEnumerator GetHighScoreAndRankingBoard()
81        {
82            scoreLabel.text = _lastScore.TextForDisplay;
83            captionLabel.text = string.Format("{0}ランキング", _board.BoardName);
84
85            //ハイスコア取得
86            {
87                highScoreLabel.text = "取得中...";
88
89                var hiScoreCheck = new YieldableNcmbQuery<NCMBObject>(_board.ClassName);
90                hiScoreCheck.WhereEqualTo(OBJECT_ID, ObjectID);
91                yield return hiScoreCheck.FindAsync();
92
93                if (hiScoreCheck.Count > 0)
94                {
95                    //既にハイスコアは登録されている
96                    _ncmbRecord = hiScoreCheck.Result.First();
97
98                    var s = _board.BuildScore(_ncmbRecord[COLUMN_SCORE].ToString());
99                    highScoreLabel.text = s != null ? s.TextForDisplay : "エラー";
100
101                    nameInputField.text = _ncmbRecord[COLUMN_NAME].ToString();
102                }
103                else
104                {
105                    //登録されていない
106                    highScoreLabel.text = "-----";
107                }
108            }
109
110            //ランキング取得
111            yield return StartCoroutine(LoadRankingBoard());
112
113            //スコア更新している場合、ボタン有効化
114            if (_ncmbRecord == null)
115            {
116                sendScoreButton.interactable = true;
117            }
118            else
119            {
120                var highScore = _board.BuildScore(_ncmbRecord[COLUMN_SCORE].ToString());
121
122                if (_board.Order == ScoreOrder.OrderByAscending)
123                {
124                    //数値が低い方が高スコア
125                    sendScoreButton.interactable = _lastScore.Value < highScore.Value;
126                }
127                else
128                {
129                    //数値が高い方が高スコア
130                    sendScoreButton.interactable = highScore.Value < _lastScore.Value;
131                }
132
133                Debug.Log(string.Format("登録済みスコア:{0} 今回スコア:{1} ハイスコア更新:{2}", highScore.Value, _lastScore.Value,
134                    sendScoreButton.interactable));
135            }
136        }
137
138
139        public void SendScore()
140        {
141            StartCoroutine(SendScoreEnumerator());
142        }
143
144        private IEnumerator SendScoreEnumerator()
145        {
146            sendScoreButton.interactable = false;
147            highScoreLabel.text = "送信中...";
148
149            //ハイスコア送信
150            if (_ncmbRecord == null)
151            {
152                _ncmbRecord = new NCMBObject(_board.ClassName);
153                _ncmbRecord.ObjectId = ObjectID;
154            }
155
156            _ncmbRecord[COLUMN_NAME] = InputtedNameForSave;
157            _ncmbRecord[COLUMN_SCORE] = _lastScore.Value;
158            NCMBException errorResult = null;
159
160            yield return _ncmbRecord.YieldableSaveAsync(error => errorResult = error);
161
162            if (errorResult != null)
163            {
164                //NCMBのコンソールから直接削除した場合に、該当のobjectIdが無いので発生する（らしい）
165                _ncmbRecord.ObjectId = null;
166                yield return _ncmbRecord.YieldableSaveAsync(error => errorResult = error); //新規として送信
167            }
168
169            //ObjectIDを保存して次に備える
170            ObjectID = _ncmbRecord.ObjectId;
171
172            highScoreLabel.text = _lastScore.TextForDisplay;
173
174            yield return StartCoroutine(LoadRankingBoard());
175        }
176
177
178        /// <summary>
179        /// ランキング取得＆表示
180        /// </summary>
181        /// <returns>The ranking board.</returns>
182        private IEnumerator LoadRankingBoard()
183        {
184            int nodeCount = scrollViewContent.childCount;
185            for (int i = nodeCount - 1; i >= 0; i--)
186            {
187                Destroy(scrollViewContent.GetChild(i).gameObject);
188            }
189
190            var msg = Instantiate(readingNodePrefab, scrollViewContent);
191
192            //2017.2.0b3の描画されないバグ暫定対応
193            MaskOffOn();
194
195            var so = new YieldableNcmbQuery<NCMBObject>(_board.ClassName);
196            so.Limit = 30;
197            if (_board.Order == ScoreOrder.OrderByAscending)
198            {
199                so.OrderByAscending(COLUMN_SCORE);
200            }
201            else
202            {
203                so.OrderByDescending(COLUMN_SCORE);
204            }
205
206            yield return so.FindAsync();
207
208            Debug.Log("データ取得 : " + so.Count.ToString() + "件");
209            Destroy(msg);
210
211            if (so.Error != null)
212            {
213                Instantiate(unavailableNodePrefab, scrollViewContent);
214            }
215            else if (so.Count > 0)
216            {
217                int rank = 0;
218                foreach (var r in so.Result)
219                {
220                    var n = Instantiate(rankingNodePrefab, scrollViewContent);
221                    var rankNode = n.GetComponent<RankingNode>();
222                    rankNode.NoText.text = (++rank).ToString();
223                    rankNode.NameText.text = r[COLUMN_NAME].ToString();
224
225                    var s = _board.BuildScore(r[COLUMN_SCORE].ToString());
226                    rankNode.ScoreText.text = s != null ? s.TextForDisplay : "エラー";
227
228//                    Debug.Log(r[COLUMN_SCORE].ToString());
229                }
230            }
231            else
232            {
233                Instantiate(notFoundNodePrefab, scrollViewContent);
234            }
235        }
236
237        public void OnCloseButtonClick()
238        {
239            closeButton.interactable = false;
240            UnityEngine.SceneManagement.SceneManager.UnloadSceneAsync("Ranking");
241        }
242
243        private void MaskOffOn()
244        {
245            //2017.2.0b3でなぜかScrollViewContentを追加しても描画されない場合がある。
246            //親maskをOFF/ONすると直るので無理やり・・・
247            var m = scrollViewContent.parent.GetComponent<Mask>();
248            m.enabled = false;
249            m.enabled = true;
250        }
251    }
252}