build method Null safety

Implementation

@override
Widget build(BuildContext context) {
  final ThemeData themeData = Theme.of(context);
  return Column(
    crossAxisAlignment: CrossAxisAlignment.start,
    children: <Widget>[
      if (groupElement.title.isNotEmpty)
        Padding(
          padding: const EdgeInsets.only(top: LARGE_SPACE),
          child: Text(
            groupElement.title,
            style: themeData.textTheme.subtitle2!.apply(color: Colors.grey),
          ),
        ),
      for (String panelId in groupElement.panelIds)
        KnowledgePanelCard(
          panel: allPanels.panelIdToPanelMap[panelId]!,
          allPanels: allPanels,
        )
    ],
  );
}