HttpClient Xamarin.Forms - xamarin

Question

I have an application that allows users to log in via facebook. I am trying to save each user to my database using my WebApi. However, I am encountering this exception error: System.NullReferenceException: Object reference not set to an instance of an object. Can anyone see what I am doing incorrectly to cause this. Thanks.
CustomerService class:
public async Task<int> AddCustomer(Customer cust)
{
var data = JsonConvert.SerializeObject(cust);
var content = new StringContent(data, Encoding.UTF8, "application/json");
client.DefaultRequestHeaders.Add("X-Giftworx-App", "Posworx");
var response = await client.PostAsync("http/my api address/api/Customer/Insert", content);
var result = JsonConvert.DeserializeObject<int>(response.Content.ReadAsStringAsync().Result);
return result;
}
Customer class:
public class Customer
{
public string Token { get; set; }
public bool Authenticated { get; set; }
public string SecretKey { get; set; }
public int StoreCustomerID { get; set; }
public string Number { get; set; }
public string Name { get; set; }
public string Surname { get; set; }
public object Address { get; set; }
public string Email { get; set; }
public string City { get; set; }
public string Region { get; set; }
public string Country { get; set; }
public string MobilePhone { get; set; }
public DateTime DOB { get; set; }
public object Phone { get; set; }
public object DeviceToken { get; set; }
public object Details { get; set; }
public object Gender { get; set; }
public bool IsError { get; set; }
public object ErrorMessage { get; set; }
public bool PhoneVerified { get; set; }
}
FacebookRender
public class FacebookRender : PageRenderer
{
CustomerService customerService;
public FacebookRender()
{
var activity = this.Context as Activity;
var auth = new OAuth2Authenticator(
clientId: "my app client's id",
scope: "",
authorizeUrl: new Uri("https://www.facebook.com/dialog/oauth/"),
redirectUrl: new Uri("https://www.facebook.com/connect/login_success.html")
);
auth.Completed += async (sender, eventArgs) =>
{
if (eventArgs.IsAuthenticated)
{
await AccountStore.Create().SaveAsync(eventArgs.Account, "FacebookProviderKey");
var accessToken = eventArgs.Account.Properties["access_token"].ToString();
var expiresIn = Convert.ToDouble(eventArgs.Account.Properties["expires_in"]);
var expiryDate = DateTime.Now + TimeSpan.FromSeconds(expiresIn);
var request = new OAuth2Request("GET", new Uri("https://graph.facebook.com/me"), null, eventArgs.Account);
var response = await request.GetResponseAsync();
var obj = JObject.Parse(response.GetResponseText());
var id = obj["id"].ToString().Replace("\"", "");
var name = obj["name"].ToString().Replace("\"", "");
Customer cust = new Customer();
cust.Token = accessToken;
cust.Name = name;
await customerService.AddCustomer(cust);
App.NavigateToProfile(string.Format(name));
}
else
{
App.NavigateToProfile("Invalid Login");
}
};
activity.StartActivity(auth.GetUI(activity));
}
}