获取错误对对象的无效对象引用和实例

Question

下面是在将数据从 GridView 导出到 CSV 时出现错误的代码，请帮助我将 csv 导出所有数据，但我没有成功导出对话框，但收到错误消息，指出对对象实例的对象引用无效

字符串 csv = string.Empty; 尝试 {

            using (SaveFileDialog sfd = new SaveFileDialog() { Filter = "csv|*.csv", ValidateNames = true })
            {
                if (sfd.ShowDialog() == DialogResult.OK)
                {

                    foreach (DataGridViewColumn column in dataGridView1.Columns)
                    {

                        csv += column.HeaderText + ',';

                    }
                    csv += "\r\n";

                    foreach (DataGridViewRow row in dataGridView1.Rows)
                    {
                        foreach (DataGridViewCell cell in row.Cells)
                        {
                            //Add the Data rows.
                            csv += cell.Value.ToString().Replace(",", ";") + ',';

                        }
                        csv += "\r\n";
                        //string folderPath = "C:\\Download\\";
                        File.WriteAllText(sfd.FileName, csv);

                    }

                }
            }

            MessageBox.Show("Exported Successfully");


        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }