用户代码未处理 System.NullReferenceException

 这段相同内容合并单元格的代码，在运行时，把滚动条拉到最底下时发生异常，发生在以下黄色语句，要怎么改？ private void dataGridView1_CellPainting(object sender, DataGridViewCellPaintingEventArgs e)
        {
            if ((e.ColumnIndex == 0 || e.ColumnIndex==1 || e.ColumnIndex==2 ||e.ColumnIndex==3) && e.RowIndex != -1)
            {
                using(Brush gridBrush = new SolidBrush(this.dataGridView1.GridColor),backColorBrush = new SolidBrush(e.CellStyle.BackColor))
                {
                    using (Pen gridLinePen = new Pen(gridBrush))
                    {
                        // 清除单元格
                        e.Graphics.FillRectangle(backColorBrush, e.CellBounds);                        // 画 Grid 边线（仅画单元格的底边线和右边线）
                        //   如果下一行和当前行的数据不同，则在当前的单元格画一条底边线
                        if (e.RowIndex < dataGridView1.RowCount - 1 && dataGridView1.Rows[e.RowIndex + 1].Cells[e.ColumnIndex].Value.ToString() !=e.Value.ToString())                            e.Graphics.DrawLine(gridLinePen, e.CellBounds.Left,e.CellBounds.Bottom - 1, e.CellBounds.Right-1 ,e.CellBounds.Bottom - 1);
                        // 画右边线
                        e.Graphics.DrawLine(gridLinePen, e.CellBounds.Right - 1,e.CellBounds.Top, e.CellBounds.Right - 1,e.CellBounds.Bottom);                        // 画（填写）单元格内容，相同的内容的单元格只填写第一个
                        if (e.Value != null)
                        {
                            if (e.RowIndex > 0 &&dataGridView1.Rows[e.RowIndex - 1].Cells[e.ColumnIndex].Value.ToString() ==e.Value.ToString())
                            { }
                            else
                            {
                                e.Graphics.DrawString((String)e.Value, e.CellStyle.Font,Brushes.Black, e.CellBounds.X + 2,e.CellBounds.Y + 5, StringFormat.GenericDefault);
                            }
                        }
                        e.Handled = true;
                    }
                }
            }        }