chore: guard on nil

http-html.go

@@ -182,7 +182,9 @@ func (r *HTWriter) WriteTo(w io.Writer) (int64, error) {
 		}
 		subject := ""
 		if s := twt.Subject(); s != nil {
-			subject = s.Tag().Text()
+			if tag, ok := s.Tag().(*lextwt.Tag); ok {
+				subject = tag.Text()
+			}
 		}
 		i, err = fmt.Fprintf(w, `
 			<article>