evaluate method Null safety

Implementation

@override
FutureOr<Evaluation> evaluate(WidgetTester tester) {
  final SemanticsNode root = tester.binding.pipelineOwner.semanticsOwner!.rootSemanticsNode!;
  Evaluation traverse(SemanticsNode node) {
    Evaluation result = const Evaluation.pass();
    node.visitChildren((SemanticsNode child) {
      result += traverse(child);
      return true;
    });
    if (node.isMergedIntoParent ||
        node.isInvisible ||
        node.hasFlag(ui.SemanticsFlag.isHidden)) {
      return result;
    }
    final SemanticsData data = node.getSemanticsData();
    // Skip node if it has no actions, or is marked as hidden.
    if (!data.hasAction(ui.SemanticsAction.longPress) &&
        !data.hasAction(ui.SemanticsAction.tap)) {
      return result;
    }
    if ((data.label == null || data.label.isEmpty) && (data.tooltip == null || data.tooltip.isEmpty)) {
      result += Evaluation.fail(
        '$node: expected tappable node to have semantic label, '
        'but none was found.\n',
      );
    }
    return result;
  }

  return traverse(root);
}