message: Use `match` statement in `Content::overlay`
  ... to improve readability a bit.
diff: @@ -195,13 +195,12 @@ where
  ) -> Option<overlay::Element<'_, Message, Renderer>> {
  if let Some(title_bar) = self.title_bar.as_mut() {
  let mut children = layout.children();
  - let title_bar_layout = children.next().unwrap();
  + let title_bar_layout = children.next()?;
  - if let Some(overlay) = title_bar.overlay(title_bar_layout) {
  - return Some(overlay);
  + match title_bar.overlay(title_bar_layout) {
  + Some(overlay) => Some(overlay),
  + None => self.body.overlay(children.next()?),
  }
  -
  - self.body.overlay(children.next()?)
  } else {
  self.body.overlay(layout)
  }
  