App crashes if location access is not given

[tvonbomhard]

On IOS:
If I remove the access for the location service via the IOS general settings and then start the app again, it crashes. It tries to execute this function

window.plugins.DGGeofencing.startMonitoringSignificantLocationChanges(
function(result) {
console.log('Geo: start monitoring significant location changes');
deferred.resolve(result);
},
function(error) {
console.log('Significant-Geo: error');
signStarted = false;
deferred.reject(error);
});

and fails with an uncaught exception in IOS like that:
* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* setObjectForKey: object cannot be nil (key: message)'

[bromeostasis]

@tvonbomhard you can fix this by just setting the message to initially be an empty string instead of nil.

Change

https://github.com/radshag/PhoneGap-Geofencing/blob/master/src/ios/src/DGGeofencing.m#L293

to

NSString* message = @"";