I cant Display data in Xamarin Forms using WCF postgresql

Question

This is my code into the wcf servis

public DataSet SelectUserDetails()

    {
        npgsqlcon.......   
        NpgsqlCommand cmd = new NpgsqlCommand("Select * from RegistrationTable", con);
        NpgsqlDataAdapter sda = new NpgsqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        sda.Fill(ds);
        cmd.ExecuteNonQuery();
        con.Close();
        return ds;
    }

And this is the xamarin forms code

public partial class MainPage : ContentPage { ServiceReference1.Service1Client objService = new ServiceReference1.Service1Client();

    public MainPage()
    {
        InitializeComponent();
        showdata();
    }

    private async void showdata()
    {

        DataSet ds = new DataSet();
        ds = await objService.SelectUserDetailsAsync();
        liste.ItemsSource = (System.Collections.IEnumerable)ds.Tables[0];
        
        
    }

this is the error