How to declare variables correctly so that there is no error?

Question

ASP.NET

Have a nice day, everyone!
Created a user control for an asp.net webforms application. It works, but an error is displayed in the studio - how do I correctly declare variables so that this error does not occur?
The essence of the error screen

C# user control

[ParseChildren(true, "Content")]
[PersistChildren(true)]
public partial class Controls_Popup : System.Web.UI.UserControl
{
    public string ppId;
    private Control _content;
    private Control _footer;

    [PersistenceMode(PersistenceMode.InnerProperty)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
    public Control Content
    {
        get { return _content; }
        set { _content = value; }
    }
    public Control Footer
    {
        get {
            if (_footer == null)
            {
                _footer = new Control();
            }
            return _footer; }
        set { _footer = value; }
    }
    protected void Page_Load(object sender, EventArgs e)
    {
        ppContent.Controls.Add((Control)_content);
        ppFooter.Controls.Add((Control)_content);
    }
}

Custom control markup

<div id="<%=ppId%>" class="popup" style="display:none">
    <div class="popup-content" style="width:300px">
        <div class="popup-btn-close" onclick="ShowPP('<%=ppId%>');">
            <img height="24" alt="" src="../../Style/img/IconGrey24DeleteLight.png" />
        </div>
        <div id="ppContent" runat="server" class="popup-content-2">
            
        </div>
        <div id="ppFooter" runat="server" class="popup-func-btns">
           
        </div>
    </div>
    <div class="popup-bg" onclick="ShowPP('<%=ppId%>');"></div>
</div>

Answer 1

Have you considered this example?
https://stackoverflow.com/a/3905584/2822609