各位求助 为什么导出数据库成外界EXCEL会出错 谢谢了

各位求助 为什么导出数据库成外界EXCEL会出错 谢谢了 
 public void ExportDataGridViewToExcel(DataGridView dataGridview1)
        {
            SaveFileDialog saveFileDialog = new SaveFileDialog();
            saveFileDialog.Filter = "Execl  files  (*.xls)|*.xls";
            saveFileDialog.FilterIndex = 0;
            saveFileDialog.RestoreDirectory = true;
            saveFileDialog.CreatePrompt = true;
            saveFileDialog.Title = "导出Excel文件到";            DateTime now = DateTime.Now;
            saveFileDialog.FileName = now.Year.ToString().PadLeft(2) + now.Month.ToString().PadLeft(2, '0') + now.Day.ToString().PadLeft(2, '0') + "-" + now.Hour.ToString().PadLeft(2, '0') + now.Minute.ToString().PadLeft(2, '0') + now.Second.ToString().PadLeft(2, '0');
            saveFileDialog.ShowDialog();            Stream myStream;
            myStream = saveFileDialog.OpenFile();
            StreamWriter sw = new StreamWriter(myStream, System.Text.Encoding.GetEncoding("gb2312"));
            string str = "";
            try
            {
                //写标题    
                for (int i = 0; i < dataGridview1.ColumnCount; i++)
                {
                    if (i > 0)
                    {
                        str += "\t";
                    }
                    str += dataGridview1.Columns[i].HeaderText;
                }
                sw.WriteLine(str);
                //写内容 
                for (int j = 0; j < dataGridview1.Rows.Count; j++)
                {
                    string tempStr = "";
                    for (int k = 0; k < dataGridview1.Columns.Count; k++)
                    {
                        if (k > 0)
                        {
                            tempStr += "\t";
                        }
                        //if (dataGridView1.Rows[j].Cells[k].Value != null)
                        //{
                        //    tempStr += dataGridView1.Rows[j].Cells[k].Value.ToString();
                        //}
                        tempStr += dataGridview1.Rows[j].Cells[k].Value.ToString();
                    }
                    sw.WriteLine(tempStr);
                }
                sw.Close();
                myStream.Close();
                MessageBox.Show("导出成功");
            }
            catch (Exception e)
            {
                MessageBox.Show(e.ToString());
            }
            finally
            {
                sw.Close();
                myStream.Close();
            }
        }//导出excel错误是：Systen.NullReferenceException:未将对象引用设置到对象实例