"Range error (index) : invalid value" using Flutter

I have two lists: the first list is called PlanExerciseslist. Its data are retrieved from Firestore and the second list is list of checkboxes… Here is the code to understand:

late List<bool> isChecked;
isChecked = List<bool>.filled(LifeStyleCubit.get(context).PlanExerciseslist.length, false);

The problem is that List<bool> isChecked doesn’t have any list yet of PlanExerciseslist.length, because PlanExerciseslist takes some time to bring the data from Firestore. So it gives me a Range error (index).

Here is my full code:

class BackPlanScreen extends StatefulWidget {
  PlanQsModel model;
  int dayNum;
  int weekNum;
  BackPlanScreen(this.model, this.dayNum, this.weekNum);

  @override
  State<BackPlanScreen> createState() => _BackPlanScreenState();
}

class _BackPlanScreenState extends State<BackPlanScreen> {
  late List<bool> isChecked;
  @override
  void initState() {
    super.initState();
    isChecked = List<bool>.filled(LifeStyleCubit.get(context).PlanExerciseslist.length, false);
  }

  @override
  Widget build(BuildContext context) {
    return BlocConsumer<LifeStyleCubit, LifeStates>(
      listener: (context, state)
      {},
      builder: (context, state)
      {
        var cubit = LifeStyleCubit.get(context);
        cubit.getPlanBackEx1(widget.model.level);
        return Scaffold(
          appBar: AppBar(
            title: Text('Exercise'),
          ),
          body: Padding(
            padding: const EdgeInsets.all(20.0),
            child: ListView.separated(
                scrollDirection: Axis.vertical,
                itemBuilder:(context, index) => BackWidget(cubit.PlanExerciseslist[index], context, index),
                separatorBuilder: (context, index) => SizedBox(height: 35,),
                itemCount: cubit.PlanExerciseslist.length
            ),
          ),
        );
      },
    );
  }

  Widget BackWidget (PlanExercisesModel model, context, index) => GestureDetector(
    onTap: () => Navigator.push(context,
        MaterialPageRoute(builder: (context) => BackPlanGifScreen(model))),
    child: Builder(
      builder: (context) {
        return Container(
          height: 100,
          width: MediaQuery.of(context).size.width,
          child: SingleChildScrollView(
            scrollDirection: Axis.horizontal,
            child: Row(
              children: [
                Checkbox(
                    value: isChecked[index],
                    activeColor: Colors.orangeAccent,
                    onChanged: (newBool) {
                      setState(() {
                        isChecked[index] = newBool!;
                        if (isChecked[index] == true)
                        {
                          LifeStyleCubit.get(context).createPlanExercisesDay(
                            name: model.name,
                            difficulty: model.difficulty,
                            bodyPart: model.bodyPart,
                            equipment: model.equipment,
                            gifURL: model.gifURL,
                            target: model.target,
                            weekNum: widget.weekNum + 1,
                            dayNum: widget.dayNum + 1,
                            id: widget.model.description,
                          );
                        }
                        if(isChecked[index] == false) {
                          LifeStyleCubit.get(context).deletePlanExercisesDay(
                              weekNum: widget.weekNum + 1,
                              dayNum: widget.dayNum + 1,
                              id: widget.model.description,
                              name: model.name
                          );
                        }
                      });
                    }
                ),
              ],
            ),
          ),
        );
      }
    ),
  );
}

How can I solve this problem? Or in another way, how can I make the List checkBox wait for like 3 seconds until the other list is fully retrieved from Firebase?