Panel.controls returns null when it is generated

Background: I am implementing a asp.net web form in .Net framework 4.7.2.

I am currently trying to retrieve list of values from generated controls in a Panel. Note that the panel is within a Modal.

Code of the aspx

<div class="modal fade" id="addLead" tabindex="-1" role="dialog" aria-labelledby="addLead" aria-hidden="true">
  <div class="modal-dialog modal-lg" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title">Add New Lead</h5>
        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
      <div class="modal-body">
        <div class="row">
            <div class="col-lg-12">
                <asp:Panel ID="panelAddAdditionalLeads" runat="server"></asp:Panel>
            </div>
        </div>
      </div>
      <div class="modal-footer">
        <asp:Button ID="btnAddNewRow" runat="server" class="btn btn-secondary mr-auto mr-auto" Text="+ Add Next Lead" OnClick="btnAddNewRow_Click" />
        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
        <asp:Button ID="btnAdd" runat="server" class="btn btn-outline-secondary" Text="Add" UseSubmitBehavior="false" OnClick="btnAdd_Click" data-dismiss="modal"/>  
      </div>
    </div>
  </div>
</div>

Controls are all generated successfully, but when clicking on button (created static), it returns null when I try to do the following (It is in the button click function [btnAdd_Click]):

List<string> currentPanelData = new List<string>();
if (panelAddAdditionalLeads.HasControls())
{
    foreach (Control control in panelAddAdditionalLeads.Controls)
    {
        if (control is TextBox)
        {
            if (!string.IsNullOrEmpty(((TextBox)control).Text))
            {
                currentPanelData.Add(((TextBox)control).Text);
            }
            else
            {
                currentPanelData.Add("");
            }
        }
        if (control is DropDownList)
        {
            currentPanelData.Add(((DropDownList)control).SelectedValue);
        }
    }
}

I have been trying to look for solutions, if there are also any other alternative ways, I would be more than happy to explore as well.

I have also made use of Page_PreInit to load the controls but it isn't working as well. This is the code used in Page_PreInit:

private void LoadControls()
        {
            List<string> currentPanelData = new List<string>();
            if (Session["CurrentDataList"] != null)
            {
                currentPanelData = (List<string>)Session["CurrentDataList"];
            }

            int loopCount = 0;
            //To load previous controls data
            if (currentPanelData.Count >= 1)
            {
                foreach (Control control in this.panelAddAdditionalLeads.Controls)
                {
                    if (loopCount < currentPanelData.Count)
                    {
                        if (control is TextBox)
                        {
                            ((TextBox)control).Text = currentPanelData[loopCount];
                            loopCount++;
                        }

                        if (control is DropDownList)
                        {
                            ((DropDownList)control).SelectedValue = currentPanelData[loopCount];
                            loopCount++;
                        }
                    }
                }
            }
        }

Thanks in advance! Meanwhile, I will also keep trying to see if there is any solutions for this...