Update JArray in JObject JSON.NET

I have two files one is a json another is xml and I need to merge both, I decided to transform the xml in json after concat/merge.

{
    "Level1": {
        "Level2": [
            {
                "id": "Chart",
                "Box": [
                    {
                        "id": "1",
                        "value": "10"
                    },
                     {
                        "id": "2",
                        "value": "20"
                    }
                ]
            }
        ]
    }
}

The second Json:

{
    "Level1": {
        "Level2": [
            {
                "id": "NameApp",
                "Box": [
                    {
                        "id": "2",
                        "value": "90"
                    },
                     {
                        "id": "3",
                        "value": "50"
                    }
                ]
            }
        ]
    }
}

OUTPUT :

{
    "Level1": {
        "Level2": [
            {
                "id": "Chart",
                "Box": [
                    {
                        "id": "1",
                        "value": "10"
                    },
                     {
                        "id": "2",
                        "value": "20"
                    }, {
                        "id": "2",
                        "value": "90"
                    },
                     {
                        "id": "3",
                        "value": "50"
                    }
                ]
            }
        ]
    }
}

XML code:

 XmlDocument doc = new XmlDocument();

        doc.Load(pathXml);
        doc.RemoveChild(doc.FirstChild);

        string jsonTextXml = JsonConvert.SerializeXmlNode(doc);

JSON code:

using (StreamReader readerJson = new StreamReader(pathJson))
        {
            jsonTextJson = readerJson.ReadToEnd();
        }

Code for Merge:

JObject o1 = JObject.Parse(jsonTextJson);

JObject o2 = JObject.Parse(jsonTextXml);

JArray box1 = o1["Level1"]["Level2"]["Box"][0] as JArray;

JArray box2 = o2["Level1"]["Level2"]["Box"][0] as JArray;

box1 = new JArray(box1.Concat(box2));

o1["Level1"]["Level2"]["Box"][0].Replace(box1);

When I wannt get the box1, I have this error: Object reference not set to an object instance.

I tested with this another way..

JArray box1 = o1["Level1"]["Level2"][0]["Box"] as JArray;

What is wrong?