c# Error: "Object Reference not set to an instance of an Object"

Question

I am trying to make a connection to sql server in an Xamarin application for android, however the application presents this error:

Object Reference not set to an instance of an Object

Connection class:

public class SqlServercon
    {
        SqlConnection conexaoSql;
        SqlConnectionStringBuilder builder = new SqlConnectionStringBuilder();

        public string CriarStringConexao()
    {
        builder.UserID = "sa";
        builder.Password = "11";
        builder.DataSource = "KATHERINE-PC/ONIPRESENTE";
        builder.InitialCatalog = "OniPresente";
        conexaoSql.ConnectionString = builder.ConnectionString;

        return builder.ToString();
    }

    public List<string> CarregaUsuarios()
    {
        List<string> lista;
        string StringConexaoSql = CriarStringConexao();
        string consulta = "SELECT login, senha from Usuarios";

        try
        {
            conexaoSql = new SqlConnection(StringConexaoSql);
            conexaoSql.Open();

            SqlDataReader rdr = null;
            SqlCommand cmd = new SqlCommand(consulta, conexaoSql);

            rdr = cmd.ExecuteReader();
            lista = new List<string>();

            while (rdr.Read())
            {
                Usuarios u = new Usuarios();
                u.Login = rdr["Login"].ToString();
                u.Senha = rdr["Senha"].ToString();
                lista.Add(u.Login + "             " + u.Senha);
            }
            return lista;
        }
        catch(Exception ex)
        {
            throw ex;
        }
        finally
        {
            conexaoSql.Close();
        }
    }
}

Mainactivity:

[Activity(Label = "OniApp", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        private ListView listview;
        private Button btnSql;
        private List<string> usuarios;

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);
            listview = FindViewById<ListView>(Resource.Id.listView1);
            btnSql = FindViewById<Button>(Resource.Id.button1);

            listview.ItemClick += ListView_ItemClick;
            btnSql.Click += btnSql_Click;

            // SetContentView (Resource.Layout.Main);
        }

        private void btnSql_Click(object sender, EventArgs e)
        {
            try
            {
                //conexao com SqlServer
                SqlServercon db = new SqlServercon();
                Toast.MakeText(this, "Acesso ao SqlServer feito com sucesso", ToastLength.Short).Show();

                //carrega a lista de string com dados de usuarios
                usuarios = db.CarregaUsuarios();

                //exibe os dados ListView
                ArrayAdapter<string> adapter = new ArrayAdapter<string>(this, Android.Resource.Layout.SimpleListItem1, usuarios);
                listview.Adapter = adapter;
            }
            catch(Exception ex)
            {
                Toast.MakeText(this, "Erro : " + ex.Message, ToastLength.Short).Show();
            }
        }

        private void ListView_ItemClick(object sender, AdapterView.ItemClickEventArgs e)
        {
            using (var dialog = new AlertDialog.Builder(this))
            {
                //exibe o usuario selecionado
                int posicao = e.Position;
                string valor = usuarios[posicao];
                dialog.SetTitle("Usuario Selecionado");
                dialog.SetMessage(valor);
                dialog.Show();
            }
        }
    }

Class Usuarios:

public class Usuarios
    {
        public int IdUsuario { get; set; }
        public string Login { get; set; }
        public string Senha { get; set; }
        public string Nome { get; set; }
        public string Chave { get; set; }
        public DateTime DtNasc { get; set; }
        public string Fone { get; set; }
        public string Email { get; set; }
        public int OAB { get; set; }
        public string Endereco { get; set; }
        public string Bairro { get; set; }
        public string CEP { get; set; }
        public int CodCidade { get; set; }
        public string CPF { get; set; }
        public string CNPJ { get; set; }
    }

Error image: