Flutter - The method 'map' was called on null

I had a similar issue, solved by wrapping it in a condition that was checking whether list had more than 0 items.

Pass an empty container while listProvices is null:

items: listProvices?.map((Provinces value) {
            return new DropdownMenuItem<Provinces>(
              value: value,
              child: new Text(value.name, style: new TextStyle(fontSize: 16.0),),
            );
          })?.toList() ?? [],