@override
Widget build(BuildContext context) {
  customers = getAllCustomers();

  return Scaffold(
     appBar: _getAppBar(),
     body: customers != null
          ? FutureBuilder(
              future: getFavouriteCustomers(), // contains IDs of favourite customers
              builder: (context, snapshot) {
                if (snapshot.connectionState == ConnectionState.done) {
                  if (snapshot.hasData) {
                    List<String> favouriteCustomersList = snapshot.data;

                    return ListView.builder(
                        itemCount: customers?.length ?? 0,
                        itemBuilder: (BuildContext context, int index) {
                          customer c = customers?.elementAt(index);

                          if (favouriteCustomersList.contains(c.id)) {
                            c.isSelected = true;
                          }

                          return ListTile(
                            title: Text(c.name),
                            trailing: Checkbox(
                                value: c.isFavourite,
                                onChanged: (newValue) {}),
                            onTap: () {
                              if (c.isSelected) {
                                setState(() {
                                  c.setFavourite(false);
                                });
                              } else {
                                setState(() {
                                  c.setFavourite(true);
                                }
                              }
                            },
                          );
                        });
                  }
                } else {
                  return CircularProgressIndicator();
                }
              })
          : Center(
              child: CircularProgressIndicator(),
            );
);