System.NullReferenceException in controller ASP.Net MVC

Working with (simple bowling game web app) where there are two text boxes and one submit button.

text box 1 = FirstRoll ,text box 2 = SecondRoll

User enter two numbers and cilck submit button then displays a score.

Issue : Getting System.NullReferenceException in Submit action method. Why frame is empty in first instance?

Controller

    [HttpPost]
    public JsonResult Submit(Frame[] _frames)
    {
        int result= 0;
        var objBowlingScore = new GameEngineService();

        foreach(var frame in _frames)
        {
            result = objBowlingScore.CalculateFrameScore(frame);
        }

        return Json(objBowlingScore);
    }

Model server side check validations are removed for code readability

public class Frame
{
    public int FrameId { get; set; }
    public int FirstRoll { get; set; }
    public int SecondRoll { get; set; }
    public int ThirdRoll { get; set; }
    public int Score { get; set; }

}

View

<p>1st Roll: @Html.EditorFor(m => m.FirstRoll)</p>
<p>2nd Roll: @Html.EditorFor(m => m.SecondRoll)</p>

<button id="submitButton" class="btn btn-primary btn-lg">Submit Score</button>

<p><label>The current frame is : </label><label id="lblFrameCount"></label></p>
<p><label>The current score is : </label><label id="lblTotalScore"></label></p>

@section DocumentReady {
<script type="text/javascript">

        var bowlingData = { "frames": [] };
        $('#submitButton').click(function (e) {
            var temp = {
                "firstroll": $("#FirstRoll").val(),
                "secondroll": $("#SecondRoll").val()
            };

            bowlingData.frames.push(temp);
            var element = this;

            $.ajax({
                url: "/Home/Submit",
                type: "POST",
                data: JSON.stringify(bowlingData),
                dataType: "json",
                traditional: true,
                contentType: "application/json; charset=utf-8",
                success: function (data) {
                    $("#lblTotalScore").text(data.Score);
                    $("#FirstRoll").val("");
                    $("#SecondRoll").val("");
                },
                error: function () {
                    alert("An error has occured!!!");
                }
            });
        });

</script>

Service

    ....
    public int CalculateFrameScore(Frame _frame)
    {
        return _frame.FirstRoll + _frame.SecondRoll + _frame.ThirdRoll;
    }

    ....