anchor.diff:            _error = err.code;
           });
         }
  -      _locationSubscription.cancel();
  +      _locationSubscription?.cancel();
  +      setState(() {
  +        _locationSubscription = null;
  +      });
       }).listen((LocationData currentLocation) {
         setState(() {
           _error = null;
  
positive.code:               margin: const EdgeInsets.only(right: 42),
                child: ElevatedButton(
                  child: const Text('Listen'),
                  onPressed: _listenLocation,
                ),
              ),
              ElevatedButton(
                child: const Text('Stop'),
                onPressed: _stopListen,
              )
            ],
          ),
  
negative.code:         _location = currentLocation;
        });
      });
      setState(() {});
    }
  
    Future<void> _stopListen() async {
      _locationSubscription?.cancel();
      setState(() {
        _locationSubscription = null;
      });
    }
  
    @override
    void dispose() {
      _locationSubscription?.cancel();
      setState(() {
        _locationSubscription = null;
      });
      super.dispose();
    }
  
    @override
  