ссылка на объект не указывает на экземпляр объекта

[kalipta]

ругается на зеленую строчку. подобных тем много, но я не пойму где и как изменить параметр null. подскажите пожалуйста

[spoiler]

namespace WindowsFormsApplication13
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            label6.Text = "";
            label8.Text = "";
            dataGridView2.Rows.Clear();
            dataGridView1.Rows.Clear();
            dataGridView3.Rows.Clear();
            int n = int.Parse(textBox1.Text);
            string[] mass1 = new string[n];
            for (int i = 0; i < n; i++)
            {
                mass1[i] = dataGridView3.Rows[i].Cells[0].Value.ToString();
                label6.Text += mass1[i];
            }
            string[] mass2 = new string[9] { "А", "Б", "В", "Г", "Д", "Е", "Ё", "Ж", "З" };
            int count1 = 0;
            int count2 = 0;
            int count3 = 0;
            int count4 = 0;
            int count5 = 0;
            int count6 = 0;
            int count7 = 0;
            int count8 = 0;
            int count9 = 0;
            dataGridView1.Rows.Clear();
            for (int i = 0; i < n; i++)
            {
                if (mass1[i] == mass2[0]) count1++;
                else if (mass1[i] == mass2[1]) count2++;
                else if (mass1[i] == mass2[2]) count3++;
                else if (mass1[i] == mass2[3]) count4++;
                else if (mass1[i] == mass2[4]) count5++;
                else if (mass1[i] == mass2[5]) count6++;
                else if (mass1[i] == mass2[6]) count7++;
                else if (mass1[i] == mass2[7]) count8++;
                else if (mass1[i] == mass2[8]) count9++;
            }
            double[] fn = new double[9];
            double[] mass3 = new double[9] { count1, count2, count3, count4, count5, count6, count7, count8, count9 };
            Array.Sort(mass3, mass2);
            for (int i = 0; i < 9; i++)
            {
                dataGridView1.Rows.Add(mass2[i], mass3[i]);
            }
            for (int i = 0; i < 9; i++)
            {
                fn[i] = Math.Round(mass3[i] / 20, 2);
                dataGridView1.Rows[i].Cells[2].Value = fn[i];
            }
            ////
            int m = 3;
            for (int i = 0; i < 9; i++)
            {
                if ((mass3[8] != 0) & (m < 4) & (mass3[8] != mass3[i]))
                    dataGridView1.Rows[8].Cells[m].Value = 1;
                dataGridView1.Rows[8].Cells[11].Value = dataGridView1.Rows[8].Cells[3].Value.ToString();
            }
            m = 4;
            for (int i = 0; i < 9; i++)
            {
                for (int k = 3; k <= m; k++)
                {
                    if (mass3[7] != 0)
                        dataGridView1.Rows[7].Cells[k].Value = 0;
                }
                if ((mass3[7] != 0) & (m < 5) & (mass3[7] != mass3[i]))
                    dataGridView1.Rows[7].Cells[m].Value = 1;
                dataGridView1.Rows[7].Cells[11].Value = dataGridView1.Rows[7].Cells[3].Value.ToString() +
 dataGridView1.Rows[7].Cells[4].Value.ToString();
            }
            m = 5;
            for (int i = 0; i < 9; i++)
            {
                for (int k = 3; k <= m; k++)
                {
                    if (mass3[6] != 0)
                        dataGridView1.Rows[6].Cells[k].Value = 0;
                }
                if ((mass3[6] != 0) & (m < 6) & (mass3[6] != mass3[i]))
                    dataGridView1.Rows[6].Cells[m].Value = 1;
                dataGridView1.Rows[6].Cells[11].Value = dataGridView1.Rows[6].Cells[3].Value.ToString() +
 dataGridView1.Rows[6].Cells[4].Value.ToString()
                + dataGridView1.Rows[6].Cells[5].Value.ToString();
            }
            m = 6;
            for (int i = 0; i < 9; i++)
            {
                for (int k = 3; k <= m; k++)
                {
                    if (mass3[5] != 0)
                        dataGridView1.Rows[5].Cells[k].Value = 0;
                }
                if ((mass3[5] != 0) & (m < 7) & (mass3[5] != mass3[i]))
                    dataGridView1.Rows[5].Cells[m].Value = 1;
                dataGridView1.Rows[5].Cells[11].Value = dataGridView1.Rows[5].Cells[3].Value.ToString() + 
dataGridView1.Rows[5].Cells[4].Value.ToString()
                + dataGridView1.Rows[5].Cells[5].Value.ToString() + dataGridView1.Rows[5].Cells[6];
            }

[/spoiler]

[Пепел Феникса]

вы же сами очистили датагриды, откуда там значения будут?