Optional group and subtitle

[mauriziopinotti]

I am using

          _items = S2Choice.listFrom<T, T>(
            source: items!,
            value: (index, item) => item,
            title: (index, item) => item.title!,
            subtitle: (index, item) => item.subtitle,
            group: (index, item) => item.group,
            meta: (index, item) => item,
          );

where item.subtitle and item.group are String?.

This fails with The return type 'String?' isn't a 'String', as required by the closure's context.

Basically, subtitle and group can either be provided or not provided, but if provided it must not be null; I need to provide it as null.

I fixed this locally and will submit a PR.