message: selector validation
diff: @@ -193,13 +193,12 @@ impl GlobalStyles {
  fn push_style_group_inner(&self, group: StyleGroup, droppable: bool) -> (u32, Vec<TaskHandle>) {
  let (rule_id_and_index, ids_lock) = self.rule_ids.add_new_id();
  + let empty_rule = [&group.selector, "{}"].concat();
  +
  + self.sheet.insert_rule_with_index(&empty_rule, rule_id_and_index).unwrap_or_else(|_| {
  + panic!("invalid CSS selector: `{}`", &group.selector);
  + });
  - let empty_rule = {
  - let mut rule = group.selector.to_string();
  - rule.push_str("{}");
  - rule
  - };
  - self.sheet.insert_rule_with_index(&empty_rule, rule_id_and_index).unwrap_throw();
  let declaration = self
  .sheet
  .css_rules()
  