type 'Null' is not a subtype of type 'LiteralTerm' in type cast

[javier-gantri]

Steps to Reproduce

HTML

Some invalid CSS like <div  style="background: red; #;">, for context, I'm trying to parse some emails sent using external tools that sometimes generate some invalid CSS selector.

`HtmlWidget` configuration

HtmlWidget(
        content,
        buildAsync: true,
        onErrorBuilder: (context, element, error) {
          return const Text('Error parsing the message!');
        },
      ),

Tesing environment

Expected results

don't show the red x icon, and discard invalid HTML and CSS.

Actual results

show a red x icon