An unhandled exception of type 'System.NullReferenceException' occurred in Ex_Banco.exe

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Ex_Banco
{
    public partial class Form1 : Form
    {
        private Conta[] contas = new Conta[3];

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.contas[0].Titular = new Cliente("Victor", 18);
            this.contas[0].Numero = 1;

            this.contas[1].Titular = new Cliente("Maria", 21);
            this.contas[1].Numero = 2;

            this.contas[2].Titular = new Cliente("José", 30);
            this.contas[2].Numero = 3;

            /*foreach (Conta c in conta)
            {
                comboContas.Items.Add(c.Titular);
            }*/

            //Depositando 250 na conta.
            this.contas[0].Deposita(250);

            //Atribuindo os valores aos TextBoxes do formulário.
            textoNumero.Text = Convert.ToString(this.contas[0].Numero);
            textoSaldo.Text = Convert.ToString(this.contas[0].Saldo);
            textoTitular.Text = contas[0].Titular.Nome;                       
        }

        private void depositar_Click(object sender, EventArgs e)
        {
            contas[0].Deposita(Convert.ToDouble(textoValor.Text));
            textoSaldo.Text = Convert.ToString(contas[0].Saldo);            
        }

        private void sacar_Click(object sender, EventArgs e)
        {
            contas[0].Saca(Convert.ToDouble(textoValor.Text));
            textoSaldo.Text = Convert.ToString(contas[0].Saldo);
        }
    }
}