System.NullReferenceException C sharp code

Question

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 private void button3_Click(object sender, EventArgs e)
        {
            
            if (dataGridView1.Rows.Count > 0)
            {
                Microsoft.Office.Interop.Excel.Application xcelApp = new              Microsoft.Office.Interop.Excel.Application();
                xcelApp.Application.Workbooks.Add(Type.Missing);
                for (int i = 1; i < dataGridView1.Columns.Count + 1; i++)
                {
                    xcelApp.Cells[1, i] = dataGridView1.Columns[i - 1].HeaderText;
                }

                for (int i = 0; i < dataGridView1.Rows.Count; i++)
                {
                    for (int j = 0; j < dataGridView1.Columns.Count; j++)
                    {
                        xcelApp.Cells[i + 2, j + 1] = dataGridView1.Rows[i].Cells[j].Value.ToString();
                    }
                    xcelApp.Columns.AutoFit();
                    xcelApp.Visible = true;
                }
            }
        }

I hae system nullreference exception. Could help me someone please?

Guys, where is my problem please?