C# identify dynamic control for string split array to Excel

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C#

I am sending data from dynamic textboxes to Excel with a for loop. I need to insert a for loop inside of the existing one to add some text that I am splitting with a '|' in cell E only for the new added rows. Below is my existing for loop. The problem I am having is when I try to declare the String to identify the dynamic textbox I want to access it gives me an error **"Object reference not set to an instance of an object."** Everything with double quotes is a dynamic textbox from the same panel. I can only assume it is because the control is dynamic and it hasn't been created yet at that point? but what is the difference between the other dynamics I am using in the existing for loop? The solution I am looking for is how to create the identifier so I can use the array.

int StartBundleRow = 10;
  for (int BndlRow = 0; BndlRow < bundleRows; BndlRow++)//add rows to spreadsheet
       {
       worksheet.Rows[StartBundleRow].Insert();
       worksheet.Rows[StartBundleRow].Font.Size = 14;
       worksheet.Rows[StartBundleRow].Font.Bold = true;
       worksheet.Rows[StartBundleRow].Interior.Color = CyberaWhite;
       worksheet.Columns["A"].Interior.Color = CyberaGrey;
       worksheet.Columns["J:XFD"].Interior.Color = CyberaGrey;
       worksheet.Cells[StartBundleRow, "C"].Interior.Color = CyberaPurple;
       worksheet.Rows[StartBundleRow].HorizontalAlignment = XlHAlign.xlHAlignLeft;
       worksheet.Cells[StartBundleRow, "D"].value =             srcBundlePanel.Controls["txtQtyBundle" + BndlRow].Text;
            worksheet.Cells[StartBundleRow, "E"].value = srcBundlePanel.Controls["txtProductNameBundle" + BndlRow].Text;

       ----HERE IS WHERE I AM WORKING----

 string DescriptionSplit = srcBundlePanel.Controls["txtProductDescBundle"].Text;
 string[] descriptionParts = DescriptionSplit.Split('|');
   for (int i = 0; i < descriptionParts.Length; i++)
       {
       worksheet.Rows[StartBundleRow].Insert();    //applies the description for the default bundle row
       worksheet.Rows[StartBundleRow].Font.Bold = false;
       worksheet.Cells[StartBundleRow++, "E"].Value = rowIndent + descriptionParts[i].Trim();

       ----TO HERE----
       }
       worksheet.Cells[StartBundleRow, "F"].value = srcBundlePanel.Controls["txtListPriceBundle" + BndlRow].Text;
       worksheet.Cells[StartBundleRow, "G"].value = srcBundlePanel.Controls["txtMaxDiscountBundle" + BndlRow].Text;
       worksheet.Cells[StartBundleRow++, "H"].value = srcBundlePanel.Controls["txtProposedPriceBundle" + BndlRow].Text;
       }