what make ex.Response returns null

Question

I'm scanning a single url that has many paths example: http://url.com/path1 to 1000. Sometimes I get a WebException but in my catch block, it will throw a NullReferenceException error if I don't use the line

if (x.Status == WebExceptionStatus.ProtocolError && x.Response != null)

So my question is this: Does code below fix the error or just ignore it?

And one more thing sometimes no error sometimes error but most off the time will error but if use the code below everything works just fine.

catch (WebException x)
{
    if (x.Status == WebExceptionStatus.ProtocolError && x.Response != null)
    {
        HttpWebResponse response = (HttpWebResponse)x.Response;
        if (response.StatusCode == HttpStatusCode.NotFound)
        {
           listBox3.Items.add(listBox1.Items[i].ToString());
        }
    }
}

Answer 1

Does code below fix the error or just ignore it

it is fine to check that response is null or not. it is the right way to go, as response can be null.

and in what cases response can be null, this is what MSDN says

If a response is available from the Internet resource, a WebResponse instance that contains the error response from an Internet resource; otherwise, null.