大家帮我看一段代码~~这段提示的错误是 未将对象引用设置到对象的实例

我做的是 将DataGridView里的数据 导出到Excel中，但是出现了这个错误
Microsoft.Office.Interop.Excel.Application objExcel = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel.Workbook objWorkbook = objExcel.Workbooks.Add(Missing.Value);
            Microsoft.Office.Interop.Excel.Worksheet objSheet = (Microsoft.Office.Interop.Excel.Worksheet)objWorkbook.ActiveSheet;
       
            objExcel.Visible = false;
            int displayColumnsCount = 1;
            for (int i = 0; i <= dataGridView1.ColumnCount - 1; i++)
            {
                if (dataGridView1.Columns[i].Visible == true)
                {
                    objExcel.Cells[1, displayColumnsCount] = dataGridView1.Columns[i].HeaderText.Trim();
                    displayColumnsCount++;
                }
            }
            for (int row = 0; row <= dataGridView1.RowCount - 1; row++)
            {
                displayColumnsCount = 1;
                for (int col = 0; col < columnscount; col++)
                {
                    if (dataGridView1.Columns[col].Visible == true)
                    {                        try
                        {
                           
                            
                          objExcel.Cells[row + 2, displayColumnsCount] = dataGridView1.Rows[row].Cells[col].Value.ToString().Trim();
                                displayColumnsCount++;
                            
                        }
                        catch (Exception ex)
                        {
                            MessageBox.Show(ex.Message);
                        }                    }                }            }
 objWorkbook.SaveAs(FileNameString, Missing.Value, Missing.Value, Missing.Value, Missing.Value, 
 Missing.Value, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlShared, Missing.Value, Missing.Value,
 Missing.Value, Missing.Value, Missing.Value); 
 
 MessageBox.Show(FileNameString + "\n\n导出完毕! ", "提示 ", MessageBoxButtons.OK, MessageBoxIcon.Information); 我感觉问题出现在 for (int row = 0; row <= dataGridView1.RowCount - 1; row++)
            {
                displayColumnsCount = 1;
                for (int col = 0; col < columnscount; col++)
                {
                    if (dataGridView1.Columns[col].Visible == true)
                    {                        try
                        {
                           
                            
                          objExcel.Cells[row + 2, displayColumnsCount] = dataGridView1.Rows[row].Cells[col].Value.ToString().Trim();
                                displayColumnsCount++;
                            
                        }
                        catch (Exception ex)
                        {
                            MessageBox.Show(ex.Message);
                        }                    }                }
这个循环中  ，请问  大家有遇到的么~~怎么解决啊