showing items of picker while consuming web service xamarin - rest

Question

i want to show the items of picker from consuming Restful webservice, but i have an error !
<Picker x:Name="natures" ItemsSource="{Binding Naturee}" SelectedItem="ItemNature"
ItemDisplayBinding="{Binding Name}"
Title="choisir votre nature de dépense"
SelectedIndexChanged="natures_SelectedIndexChanged"/>
my modal PickerModelNature
public class PickerModelNature
{
public class NatureD
{
public string Label;
}
public class ResponseDataN
{
public RootModel Data;
}
public class RootModel : INotifyPropertyChanged
{
List<NatureD> natureList;
[JsonProperty("natureList")]
public List<NatureD> NatureList
{
get { return natureList; }
set
{
if (natureList != value)
{
natureList = value;
OnPropertyChanged();
}
}
}
NatureD itemNature;
public NatureD ItemNature
{
get { return itemNature; }
set
{
if (itemNature != value)
{
itemNature = value;
OnPropertyChanged();
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = "")
{
var changed = PropertyChanged;
if (changed == null)
return;
changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
}
and the xaml.cs
public partial class CreerDepense : ContentPage
{
public CreerDepense()
{
InitializeComponent();
this.BindingContext = new RootModel();
GetExpenseNature();
} private async void GetExpenseNature()
{
HttpClient httpClient = new HttpClient();
httpClient.BaseAddress = new Uri("http://192.168.1.6:3000/api/adepApi/GetExpensesNatureList");
httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
HttpResponseMessage response = await httpClient.GetAsync("http://192.168.1.6:3000/api/adepApi/GetExpensesNatureList");
var content = await response.Content.ReadAsStringAsync();
ResponseDataN EL = JsonConvert.DeserializeObject<ResponseDataN>(content);
// var Items = JsonConvert.DeserializeObject<List<NatureD>>(content);
//listexpense.ItemsSource = Items;
natures.ItemsSource = EL.Data.NatureList;
}
the error is:
Java.Lang.NullPointerException: 'Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference'
what should i do ?