asp.net find control inside runat div

I have this div in my markup

<form id="form1" runat="server">
    <div class="mt-5">
        <div id="d" runat="server"></div>
    </div>
</form>

I then add to it some html elements by codebehind:

string[] parole = { "morning", "good", "StackOverflow!" };
string[] ordinalID = { "2", "1", "3" };
string[] images = { "images/img-1.jpg", "images/img-2.jpg", "images/img-3.jpg" };

HtmlGenericControl unorderedList = new HtmlGenericControl("ul");
unorderedList.Attributes["class"] = "sortable-list";
unorderedList.Attributes["id"] = "sortable";
unorderedList.ClientIDMode = ClientIDMode.Static;


Int16 i = -1;
foreach (string parola in parole)
{
    ++i;
   
    HtmlGenericControl listItem = new HtmlGenericControl("li");
    listItem.Attributes["class"] = "item";
    listItem.Attributes["id"] = ordinalID[i];
    listItem.Attributes["draggable"] = "true";
    listItem.ClientIDMode = ClientIDMode.Static;

    HtmlGenericControl div = new HtmlGenericControl("div");
    div.Attributes["class"] = "details";

    HtmlGenericControl img = new HtmlGenericControl("img");
    img.Attributes["src"] = images[i];

    HtmlGenericControl span = new HtmlGenericControl("span");
    span.InnerText = parola;

    HtmlGenericControl info = new HtmlGenericControl("i");
    info.Attributes["class"] = "uil uil-draggabledots";

    div.Controls.Add(img);
    div.Controls.Add(span);
    listItem.Controls.Add(div);
    listItem.Controls.Add(info);
    unorderedList.Controls.Add(listItem);
}


d.Controls.Add(unorderedList);

At some point I need to retrieve the listItem element to change its class attribute, and I'm trying to do like this:

HtmlGenericControl ul = (HtmlGenericControl)d.FindControl("sortable");
HtmlGenericControl listItem = (HtmlGenericControl)ul.FindControl(ordinalID);
listItem.Attributes["class"] = "item bg-danger";

But gives me a null reference exception on ul element... how can I achieve what I want? Thanks!

This may not be a real answer, but shows why setting ID as attribute does not work and should be set as property.

A few modifications to the original code:

<form id="form1" runat="server">
    <div>
        <div class="mt-5">
            <div id="d" runat="server"></div>
        </div>
        <asp: Button ID="btn1" runat="server" Text="Check" OnClick="btn1_Click" />
        <asp: Label id="Lbl1" runat="server"></asp: Label>
    </div>
</form>

creating elements:

string[] parole = { "morning", "good", "StackOverflow!" };
string[] ordinalID = { "2", "1", "3" };
string[] images = { "images/img-1.jpg", "images/img-2.jpg", "images/img-3.jpg" };

void create_elements()
{
    HtmlGenericControl unorderedList = new HtmlGenericControl("ul")
    {
        ID = "sortable",
        ClientIDMode = ClientIDMode.Static
    };
    unorderedList.Attributes.Add("class", "sortable-list");

    Int16 i = -1;
    foreach (string parola in parole)
    {
        ++i;

        HtmlGenericControl listItem = new HtmlGenericControl("li")
        {
            ID = ordinalID[i],
            ClientIDMode = ClientIDMode.Static
        };
        listItem.Attributes.Add("class" , "item");
        listItem.Attributes.Add("draggable" , "true");

        HtmlGenericControl div = new HtmlGenericControl("div");
        div.Attributes.Add("class", "details");
  
        HtmlGenericControl img = new HtmlGenericControl("img");
        img.Attributes.Add("src" , images[i]);

        HtmlGenericControl span = new HtmlGenericControl("span")
        {
            InnerText = parola
        };

        HtmlGenericControl info = new HtmlGenericControl("i");
        info.Attributes.Add("class", "uil uil-draggabledots");

        div.Controls.Add(img);
        div.Controls.Add(span);
        listItem.Controls.Add(div);
        listItem.Controls.Add(info);
        unorderedList.Controls.Add(listItem);
    }
    d.Controls.Add(unorderedList);
}

check:

protected void btn1_Click(object sender, EventArgs e)
{
    HtmlGenericControl ul = (HtmlGenericControl)d.FindControl("sortable");
    HtmlGenericControl listItem = (HtmlGenericControl)ul.FindControl(ordinalID[0]);
    listItem.Attributes.Add("class", "item bg-danger");
    Lbl1.Text=listItem.Attributes["class"];
}

inspecting shows the findcontrol now can find elements from ID:

result: