Utf8Jsonでキーを網羅していないクラスで長いJSONをデシリアライズしようとするとNullReferenceExceptionになる例

error.json

{
  "success": 0,
  "data": {
    "code": 10000
  }
}

example.json

{
  "success": 1,
  "data": {
    "transactions": [
      {
        "transaction_id": 12189864,
        "side": "buy",
        "price": "841708",
        "amount": "0.0020",
        "executed_at": 1528295539542
      },
      {
        "transaction_id": 12189863,
        "side": "buy",
        "price": "841707",
        "amount": "0.0080",
        "executed_at": 1528295539500
      }
    ]
  }
}

ParseBigJsonWithUtf8Json.cs

using UniRx;
using UnityEditor;
using UnityEngine;
using Utf8Json;

public class ParseBigJsonWithUtf8Json : MonoBehaviour
{
    public void ParseSmallJson()
    {
        ObservableWWW.Get("https://public.bitbank.cc/btc_jpy/transactions")
            .Do(Debug.Log)
            .Select(JsonSerializer.Deserialize<Response>)
            .Subscribe(response => Debug.Log(JsonSerializer.ToJsonString(response)));
    }

    public void ParseBigJson()
    {
        ObservableWWW.Get("https://public.bitbank.cc/btc_jpy/transactions/20180603")
            .Do(Debug.Log)
            .Select(JsonSerializer.Deserialize<Response>)
            .Subscribe(response => Debug.Log(JsonSerializer.ToJsonString(response)));
    }
}

// JSONに含まれるdataを持たないクラス
public class Response
{
    public readonly int success; // 0 or 1

    public Response(int success)
    {
        this.success = success;
    }
}

// インスペクタにボタンを表示する用
[CustomEditor(typeof(ParseBigJsonWithUtf8Json))]
public class ParseBigJsonWithUtf8JsonEditor : Editor
{
    private ParseBigJsonWithUtf8Json monoBehaviour => (ParseBigJsonWithUtf8Json) target;

    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI();

        if (GUILayout.Button("Parse Small JSON"))
        {
            monoBehaviour.ParseSmallJson();
        }

        if (GUILayout.Button("Parse Big JSON"))
        {
            monoBehaviour.ParseBigJson();
        }
    }
}

Runtime Error

NullReferenceException: Object reference not set to an instance of an object
UniRx.Stubs.<Throw>m__1 (System.Exception ex) (at Assets/UniRx/Plugins/Observer.cs:495)
UniRx.Observer+Subscribe`1[T].OnError (System.Exception error) (at Assets/UniRx/Plugins/Observer.cs:172)
UniRx.Operators.SelectObservable`2+Select[T,TR].OnNext (T value) (at Assets/UniRx/Plugins/Operators/Select.cs:88)
UniRx.Operators.DoObservable`1+Do[T].OnNext (T value) (at Assets/UniRx/Plugins/Operators/Do.cs:56)
UniRx.Operators.FromCoroutineObservable`1+FromCoroutine[T].OnNext (T value) (at Assets/UniRx/Plugins/UnityEngineBridge/Operators/FromCoroutine.cs:49)
UniRx.ObservableWWW+<FetchText>c__Iterator1.MoveNext () (at Assets/UniRx/Plugins/UnityEngineBridge/ObservableWWW.cs:265)
UnityEngine.SetupCoroutine.InvokeMoveNext (System.Collections.IEnumerator enumerator, System.IntPtr returnValueAddress) (at /Users/builduser/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)