HTML Agility pack throw in "An unhandled exception of type 'System.ArgumentNullException' occurred in mscorlib.dll" Exception - html-agility-pack

Question

I'm going to parsing a website content Website HTML Content. I use follow codes:
private async void loadButton_Click(object sender, RoutedEventArgs e)
{
string address = "http://www.iiees.ac.ir/fa/eqcatalog";
loadButton.IsEnabled = false;
//string htmlCode = await DownloadStringAsync(address, Encoding.UTF8);
string htmlCode = await GetDataAsync(address, Encoding.UTF8);
paragraph1.Inlines.Add(new Run(htmlCode));
loadButton.IsEnabled = true;
}
private async Task<string> GetDataAsync(string address, Encoding encoding)
{
string htmlCode = await DownloadStringAsync(address, encoding);
List<string> options = new List<string>();
HtmlDocument doc = new HtmlDocument();
doc.LoadHtml(htmlCode);
foreach (var table in doc.DocumentNode.SelectNodes("//table"))
{
if (table.Id == "CatalogForm")
{
foreach (HtmlNode row in table.SelectNodes("tr"))
{
if (row.Attributes.Any(a => a.Value == "hidecircle"))
{
foreach (HtmlNode td in row.SelectNodes("TD"))
{
foreach (HtmlNode option in td.SelectNodes("option"))
{
options.Add(option.InnerText);
}
}
}
}
}
}
var sb = new StringBuilder();
options.ForEach(str => sb.AppendLine(str));
return sb.ToString();
}
But throw in "An unhandled exception of type 'System.ArgumentNullException' occurred in mscorlib.dll" exception.
For more information, I upload my project here.
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Answer 1

I created a sample from your code, and I found the following:
I am not particularly familiar with xpath, but seems it is case sensitive. Searching for "TD" will return nothing, while searching for "td" will.
SelectNodes will return null whenever nothing was found, and a foreach that depends on it will always throw an exception. So it is better to check whether the result of SelectNodes is not null before trying to loop over it.
foreach (HtmlNode td in row.SelectNodes("td"))
{
var ops = td.SelectNodes("option");
if (ops != null)
{
foreach (HtmlNode option in ops)
{
options.Add(option.InnerText);
}
}
}