Asp.net mvc 在mvc中使用表单，出现错误null异常

请帮帮我。我编写了以下代码：

@using (Html.BeginForm("send", "default", FormMethod.Post))
{
    <input type="text" id="txtNumber" />
    <input type="submit" value="submit 1" />
    <input type="submit" value="submit 2" />
}

但这个错误：（ 异常详细信息：System.NullReferenceException:对象引用未设置为对象的实例。>string\u data=frm[“txtNumber”]；.Tnqs请提供帮助

@using (Html.BeginForm("send", "default", FormMethod.Post))
{
    <input type="text" id="txtNumber" />
    <button type="submit" name="btn" value="btn1">submit 1</button>
    <button type="submit" name="btn" value="btn2">submit 2</button>
}

@using (Html.BeginForm("send", "default", FormMethod.Post))
{
    <input type="text" id="txtNumber" />
    <button type="submit" name="btn" value="btn1">submit 1</button>
    <button type="submit" name="btn" value="btn2">submit 2</button>
}

[HttpPost]
public HtmlString send(FormCollection frm)
{
    string _data = frm["txtNumber"];

    string btn = frm["btn"];
    if (btn == "btn1")
    {
        // submit 1 was clicked
    }
    else 
    {
        // submit 2 was clicked
    }

    return new HtmlString(_data);
}