[Flutter] Flutter/Dart: Function return type for returning a ShowDialog based on AsyncValue...

I want to create a function for my app that goes through an Async List to check if an item was previously bought, and show a dialog box if it has been. Since Asyncvalue.when() has data, error and loading parameters, Future Type of showDialog creates return type conflict with CircularProgressIndicator or a Error Text message I could possibly show. Can you let me know if his approach below is faulty, or any suggestions?

Future<Void> itemPreviouslyBought(BuildContext context, Item newItem, AsyncValue<List<Item>> currentItemList) {
return currentItemList.when(
data: (item) {
final matchedElement = item.firstWhere(
(element) =>
element.productName.toLowerCase() ==
newItem.productName.toLowerCase(),
);

return showDialog<void>(
context: context,
barrierDismissible: false,
builder: (BuildContext context) {
return AlertDialog(
title: const Text('Item Previously Bought!'),
content: SingleChildScrollView(
child: ListBody(
children: <Widget>[
Text("You have bought this item before."),
],
),
),
actions: <Widget>[
TextButton(
child: const Text('Okay'),
onPressed: () {
Navigator.of(context).pop();
},
),
],
);
},
);
},
loading: () => const Center(child: CircularProgressIndicator()),
error: (e, st) => Center(child: Text(e.toString())),
);
}


Error message is following Center widget isn't returnable from Future<InvalidType> function.

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