Исключение при выполнении кода "Object reference not set to an instance of an object" - C#

string[] array_string = new string[5];
        byte b_b = 0;
        private void button14_Click(object sender, EventArgs e)
        {
            try
            {
                string str = textBox19.Text;
                array_string[0 + b_b] = str;
                b_b++;
                label26.Text = b_b.ToString();
                textBox19.Text = "";
            }
            catch (Exception exception)
            {
                MessageBox.Show(exception.Message);
            }
        }
        private void button13_Click(object sender, EventArgs e)
        {
            try
            {
                Array.Sort<string>(array_string);
                foreach (string s in array_string)
                {
                    textBox17.AppendText(s.ToString() + "; ");
                    textBox17.Text += Environment.NewLine;
                }
            }
            catch (Exception exception)
            {
                MessageBox.Show(exception.Message);
            }
        }