"itemBuilder -> Container" is not subtype of "itemBuilder -> Widget"

Question

I am trying to pass an itemBuilder to another widget. Here's the isolated problematic widget to reproduce the error:

class TestWidget<ITEM> extends StatefulWidget {
  final List<ITEM> items;
  final Widget Function(int index, ITEM item) itemBuilder;

  const TestWidget({
    Key? key,
    required this.items,
    required this.itemBuilder,
  }) : super(key: key);

  @override
  _TestWidgetState createState() => _TestWidgetState<ITEM>();
}

class _TestWidgetState<ITEM> extends State<TestWidget> {
  // -
  @override
  Widget build(BuildContext context) {
    return widget.itemBuilder(0, widget.items[0]);
  }
}

Here's how I try to use it:

final items = ["apple", "banana"];
return TestWidget<String>(
  items: items,
  itemBuilder: (index, item) => Container(color: Colors.green, child: Text(item)),
);

This results in following error:

The following _TypeError was thrown building TestWidget(dirty, state: _TestWidgetState#2d36b): type '(int, String) => Container' is not a subtype of type '(int, dynamic) => Widget'

What am I missing here? I am new to Dart and Flutter.