"Индекс находится вне границ массива" - C#

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

namespace WindowsFormsApplication18
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        Fil ur = new Fil();
 
        private void button1_Click(object sender, EventArgs e)
        {
            listBox1.Items.Clear();
            ur.Creation(textBox1);
        }
 
        class Fil
        {
 
            public void Creation(TextBox m)
            {
                File.WriteAllText("doce.txt", m.Text, Encoding.UTF8);
 
            }
        }
 
            private void button4_Click(object sender, EventArgs e)
            {
 
                textBox1.Text = File.ReadAllText("doce.txt", Encoding.UTF8);
            }
 
            private void button6_Click(object sender, EventArgs e)
            {
                {
                   File. WriteAllText("doce.txt", textBox1.Text, Encoding.UTF8);

                     string input="";
                    textBox1.Text = input;
                    listBox1.Items.Clear();
                    
                    string[] str = input.Split(new Char[] { ' ', ',', '.', ':', '!', '?', ';' }, StringSplitOptions.RemoveEmptyEntries);
                    int maxlen = 0, index = 0;
                    for (int i = 0; i < str.Length; i++)
                    {
                        if (str[i].Length > maxlen)
                        {
                            maxlen = str[i].Length;
                             index = i;
                        }
                    }
                    listBox1.Items.Add("Самое длинное слово: " +str[index]);
 
                }
            }
 
        }
    }