So thanks to @pskink I have a working version... using a record makes it quite need although I don't think that is the thing that actually fixed it - rather using a separate future returned from _loadData and then adding a load of ? and ! to override nulls - but I just don't have time to do all the differential diagnosis to see exactly what fixed what (feel free to add comments to make my code more robust).

As an aside I like the fact that retrieving elements from records makes my code look a bit more like Perl :)

import 'package:flutter/material.dart';
import 'load_xml.dart';

class MyApp extends StatefulWidget {
  const MyApp({super.key});
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  final String xmlAssetPath = 'data.xml';
  
  Future<(DataTree, Group)>? initFuture;
  DataTree? dataTree;
  Category? selectedCategory; // = Category(name: 'not_set', groupNames: []); 
  Group? selectedGroup;
  // Group selectedGroup = Group(name: "not_set", items: []);
  String? groupName; // ="Citrus";
  Item? selectedItem; // = Item(name: "Orange");
  int selectedItemIndex = 0;

  Future<(DataTree, Group)> _loadData() async {
    dataTree = await loadXml(xmlAssetPath); 
    debugPrint ("Loading data tree...");
    for (var category in dataTree!.categories) {
      debugPrint ("Loading category ${category.name}...");
      if ((category.isDefault ?? false) && (category.defaultGroupName != null)) {
         debugPrint ("Setting groupName to ${category.defaultGroupName}...");
         groupName = category.defaultGroupName!;
       }
    } 
    debugPrint ("Loading selected group $groupName..."); 
    selectedGroup = await loadGroup(xmlAssetPath, groupName!); 
    for (var item in selectedGroup!.items) {
      debugPrint ("Loading item ${item.name}...");
      if (item.name == selectedGroup!.defaultItemName) {
         selectedItem = item;
         selectedItemIndex = selectedGroup!.items.indexOf(selectedItem!);
      }
    }
    return (dataTree!, selectedGroup!);
  }

  @override 
  void initState() { 
    super.initState(); 
    initFuture = _loadData();
  }


  Widget build(BuildContext context) {
    return FutureBuilder<(DataTree, Group)> (
        future: initFuture,
        builder: (BuildContext context, AsyncSnapshot<(DataTree, Group)> snapshot) {
          if (snapshot.hasData) {
           return MaterialApp (
             title: snapshot.data!.$2.name,
             home: ListView.builder(
              itemBuilder: (context, index) => Card(
                key: ValueKey(snapshot.data!.$2.items[index].name),
                child: ListTile(
                  title: Text(snapshot.data!.$2.items[index].name),
                  subtitle: Text((snapshot.data!.$2.items[index].name==selectedItem!.name) ? "Selected" : "Not selected"),
                ),
              ),
              itemCount: snapshot.data!.$2.items.length,
            )
           );
          } else {
            return Container();
          }
        }
    );
  }
}