Xamarin的.加载类型时发生typeeloadeexception错误

我正在使用便携式Xamarin表单-跨平台。在xamarin中有两个项目Android和IOS

下面的代码是完美的工作时，我在安卓应用程序运行。但同样的代码在IOS中不起作用。

它给出错误"失败发生时加载类型" on line "var response = await client.Execute(request);"

下面是我的代码

public async Task<ResponseGeneralObject<TokenModel>> LoginUser(LoginModel loginModel)
{
    ResponseGeneralObject<TokenModel> result = new ResponseGeneralObject<TokenModel>();
    try
    {
        var restUrl = App.URLServicePath + "token";
        var client = new RestClient(restUrl);
        TokenModel model = new TokenModel();
        var request = new RestRequest("", Method.POST);
        request.AddParameter("grant_type", "password");
        request.AddParameter("username", loginModel.UserName);
        request.AddParameter("password", loginModel.Password);
        var response = await client.Execute(request);
        var content = response.Content;
        if (response.StatusCode == System.Net.HttpStatusCode.OK)
        {
            result.data = JsonConvert.DeserializeObject<TokenModel>(content);
            // model = new JavaScriptSerializer().Deserialize<TokenModel>(response.Result.Content);
            result.iserror = false;
        }
        else
        {
            result.iserror = true;
            result.message = "Internal error!";
        }
    }
    catch (Exception ex)
    {
        result.iserror = true;
        result.message = ex.Message;
    }
    return result;
}