build method Null safety

Implementation

@override
Widget build(BuildContext context) {
  final List<Widget> elementWidgets = <Widget>[];
  elementWidgets.add(KnowledgePanelSummaryCard(panel));
  for (final KnowledgePanelElement element
      in panel.elements ?? <KnowledgePanelElement>[]) {
    elementWidgets.add(
      Padding(
        padding: const EdgeInsets.only(top: VERY_SMALL_SPACE),
        child: KnowledgePanelElementCard(
          knowledgePanelElement: element,
          allPanels: allPanels,
        ),
      ),
    );
  }
  return Column(
    crossAxisAlignment: CrossAxisAlignment.start,
    children: elementWidgets,
  );
}