C＃代码中的错误：对象引用未设置为对象的实例

Question

我正在动态创建表和一些控件，因此在生成错误时：对象引用未设置为对象的实例。在“tb.Text = Request.Form["TextBoxRow_" + i + "Col_" + j+1];”所以如何防止这种情况？

private void SetPreviousData(int rowsCount, int colsCount)
        {
            Table table = (Table)Page.FindControl("Table1");
            if (table != null)
            {
                for (int i = 0; i < rowsCount; i++)
                {
                    //DropDownList drList = (DropDownList)table.Rows[i].Cells[0].FindControl("DropDownRow_" + i + "Col_" + i);
                    //drList.Text = Request.Form["DropDownRow_" + i + "Col_" + i];
                    for (int j = 0; j < colsCount-1; j++)
                    {
                        TextBox tb = (TextBox)table.Rows[i].Cells[j].FindControl("TextBoxRow_" + i + "Col_" + j+1);                        
                        tb.Text = Request.Form["TextBoxRow_" + i + "Col_" + j+1];
                        string text = tb.Text;
                        string id = tb.ID;
                    }
                }
            }
        }
        private void GenerateTable(int rowsCount)
        {
            Table table = new Table();
            TableHeaderRow header = new TableHeaderRow();
            TableHeaderCell headerTableCell1 = new TableHeaderCell();            
            table.ID = "Table1";
            Page.Form.Controls.Add(table);
            const int colsCount = 4;

            headerTableCell1.Text = "Item Name";
            header.Cells.Add(headerTableCell1);
            table.Rows.Add(header); 
            for (int i = 0; i < rowsCount; i++)
            {
                TableCell cell = new TableCell();
                TableRow row = new TableRow();
                DropDownList drList = new DropDownList();
                drList.ID = "DropDownRow_" + i + "Col_" + i;

                cell.Controls.Add(drList);

                row.Cells.Add(cell);
                for (int j = 1; j < colsCount; j++)
                {             

                    TextBox tb = new TextBox();                    
                    tb.ID = "TextBoxRow_" + i + "Col_" + j;
                    cell.Controls.Add(tb);
                    row.Cells.Add(cell);
                }               

            }
             // Creating a header row            

            SetPreviousData(rowsCount, colsCount);
            rowsCount++;
            ViewState["RowsCount"] = rowsCount;
        }