dynamic checkboxlist creation and geting selected value

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C#

ASP.NET

The below code is used to create a dynamic checkbox list:

CheckBoxList SubCategoryChkboxLst;
protected void CategoryChkboxLst_SelectedIndexChanged(object sender, EventArgs e)
{
  for (int i = 0; i < CategoryChkboxLst.Items.Count; i++)
  {
    if (CategoryChkboxLst.Items[i].Selected == true)
    {
      objdc.ItemID = Convert.ToInt32(CategoryChkboxLst.Items[i].Value);
      DataTable dt = new DataTable();
      dt = objbl.GetSubCategories(ref objdc);
                 
      SubCategoryChkboxLst = new CheckBoxList();
      SubCategoryChkboxLst.DataSource = dt;
      SubCategoryChkboxLst.ID = "SubCategoryChkboxLst_" + objdc.ItemID;
      SubCategoryChkboxLst.DataTextField = "SUB_ITEM_NAME";
      SubCategoryChkboxLst.DataValueField = "SUB_ITEM_ID";
      SubCategoryChkboxLst.DataBind();
      SubCategoryChkboxLst.RepeatColumns = 5;
      SubCategoryChkboxLst.RepeatDirection = RepeatDirection.Horizontal;
      //lblYear.ID = "lbl_" + li.Value;

      //adding to panel for appearing in specified location
      SubCategoryPnl.Controls.Add(SubCategoryChkboxLst);
      SubCategoryPnl.Controls.Add(new LiteralControl("<br>"));
    }
  }
}

I have a problem in getting value & finding control in save button.When I find the control, I am getting null

protected void SaveImgbtn_Click(object sender, ImageClickEventArgs e)
{
  try
  {
    if (ProductNameCmbBox.SelectedIndex > 0)
    {             
      //karthik written  code
      for (int i = 0; i < CategoryChkboxLst.Items.Count; i++)
      {
        if (CategoryChkboxLst.Items[i].Selected == true)
        {
          objdc.ItemID = Convert.ToInt32(CategoryChkboxLst.Items[i].Value);
          string stId="SubCategoryChkboxLst_"+objdc.ItemID;
          CheckBoxList cklt = (CheckBoxList)Page.FindControl(stId);
          //here I am getting null ie cklt is null

          foreach (ListItem item in cklt.Items)
          {
            if (item.Selected)
            {
              objdc.subcategory += item.Value;
              //string Activities += item.Value;
            }
          }
        }
      }
      // code end

      objdc.insertedby = Session["UserId"].ToString();
      int result = objbl.InsertProductscheme(ref objdc);
      if (result == 1)
      {
        GridBindMethod();
        Literal li = new Literal();
        li.Text = "<script>alert('Data Inserted Sucessfully');</script>";
        Page.Controls.Add(li);
        Clearfields();
      }
      else
      {
        Literal li = new Literal();
        li.Text = "<script>alert('Insertion Failed');</script>";
        Page.Controls.Add(li);
      }
    }
  }
  catch (Exception ex)
  {
    log.Error(ex.Message, ex);
  }
}