anchor.diff:    final Location location = Location();
   
     LocationData? _location;
  -  late StreamSubscription<LocationData> _locationSubscription;
  +  StreamSubscription<LocationData>? _locationSubscription;
     String? _error;
   
     Future<void> _listenLocation() async {
  
positive.code:         _location = currentLocation;
        });
      });
    }
  
    Future<void> _stopListen() async {
      _locationSubscription.cancel();
    }
  
    @override
  
negative.code:               margin: const EdgeInsets.only(right: 42),
                child: ElevatedButton(
                  child: const Text('Listen'),
                  onPressed:
                      _locationSubscription == null ? _listenLocation : null,
                ),
              ),
              ElevatedButton(
                child: const Text('Stop'),
                onPressed: _locationSubscription != null ? _stopListen : null,
              )
            ],
          ),
  