Valid value range is empty: 0 - flutter

Question

my function search for a specifies pharmacy's and add these to my List of Map but my list still empty in the return
Future<List<Map>> geto(String s) async{
var user_position = await _locationService1.GetCurrentPosition();
print(user_position.latitude) ;
print(user_position.longitude) ;
List <Map> pp =[] ;
await FirebaseFirestore.instance.collection('pharma')
.get().then((value) => {
value.docs.forEach((pharma) async{
print(pharma.id) ;
await FirebaseFirestore.instance.collection('pharma')
.doc(pharma.id)
.collection('med')
.get().then((med) => {
med.docs.forEach((element) async{
if(element['qtc'] >0 && element.id.toString().toLowerCase()==s.toLowerCase()){
print(element.data()) ; // printer le nom de med
print(pharma['nom']) ; // printer la pharma de med avant
// print(pharma['latitude']) ; // acceder a un champ de pharma
var d = Geolocator.distanceBetween(
(pharma)['latitude'],
(pharma)['longitude'],
user_position.latitude,
user_position.longitude,
);
pp.add({'id' : pharma.id , 'distance' : d}) ;
print('Cest la list felicitation $pp') ;
}
})
});
})
})
;
print("--------------------...........................${pp.length}") ;
return pp ;
}
-I tried Future Builder and pass the function as the futur argument but it's didn't work and return a red screen which tell me that the list is empty