Fix onLinkTap in selectable mode not working due to null anchorKey

[tneotia]

Fixes #761

[noorhossain]

not work for me :

subtitle: Html(
              data: user.bangla_trans.replaceAll("\\n", "<br>"),
              onAnchorTap: (url, renderContext, attrs, element) async {
                print(element.toString());
                // Handle link tapped...
                if (url != null && element?.localName == "a") {
                  print(element.toString());
                  await showAlertDialog(context, element.toString());
                }
              },