Change in PopulateMenu Method like below.

C#

private void PopulateMenu(DataTable dt, int parentMenuId, MenuItem parentMenuItem)
{
    string currentPage = Path.GetFileName(Request.Url.AbsolutePath);
    foreach (DataRow row in dt.Rows)
    {
        MenuItem menuItem = new MenuItem
        {
            Value = row["MenuId"].ToString(),
            Text = row["Title"].ToString(),
            NavigateUrl = row["Url"].ToString(),
            Selected = row["Url"].ToString().EndsWith(currentPage, StringComparison.CurrentCultureIgnoreCase)
        };
        if (parentMenuId == 0)
        {
            Menu1.Items.Add(menuItem);
            DataTable dtChild = this.GetData(int.Parse(menuItem.Value));
            PopulateMenu(dtChild, int.Parse(menuItem.Value), menuItem);
        }
        else
        {
            parentMenuItem.ChildItems.Add(menuItem);
                 
            if (parentMenuId > 0)
            {
                DataTable dtChild = this.GetData(int.Parse(menuItem.Value));
                PopulateMenu(dtChild, int.Parse(menuItem.Value), menuItem);
            }
        }
    }
}

VB.Net

    Private Sub PopulateMenu(dt As DataTable, parentMenuId As Integer, parentMenuItem As MenuItem)
        Dim currentPage As String = Path.GetFileName(Request.Url.AbsolutePath)
        For Each row As DataRow In dt.Rows
            Dim menuItem As New MenuItem() With { _
             .Value = row("MenuId").ToString(), _
             .Text = row("Title").ToString(), _
              .NavigateUrl = row("Url").ToString(), _
             .Selected = row("Url").ToString().EndsWith(currentPage, StringComparison.CurrentCultureIgnoreCase) _
            }
            If parentMenuId = 0 Then
                Menu1.Items.Add(menuItem)
                Dim dtChild As DataTable = Me.GetData(Integer.Parse(menuItem.Value))
                PopulateMenu(dtChild, Integer.Parse(menuItem.Value), menuItem)
            Else
                parentMenuItem.ChildItems.Add(menuItem)
                If parentMenuId > 0 Then
                    Dim dtChild As DataTable = Me.GetData(Integer.Parse(menuItem.Value))
                    PopulateMenu(dtChild, Integer.Parse(menuItem.Value), menuItem)
                End If
            End If
        Next
    End Sub