watchLocation method
The function watchLocation checks if a location stream is already added and returns the location
if available, otherwise throws an error.
@returns The method watchLocation() returns a Future<String?>.
Implementation
@override
Future<String?> watchLocation() async {
if (_locationStream == null) {
final returnVal =
await methodChannel.invokeMethod<String>('watchLocation');
return returnVal;
} else {
throw ("Location watch already added");
}
}