Fixes issue #251.

That exception throw cause addAnnotation: sended nil-value annotation.
This method called from locationManager:didUpdateToLocation:fromLocation:
and there sended self.userLocation. Therefore self.userLocation
is already nil.
If we look at begining of this method we see call of delegate method
didUpdateUserLocation:. There we call showsUserLocation: and if we sent
NO as param, then RMMapView set self.userLocation = nil. Therefore
self.userLocation is already nil when we call addAnnotation: in
locationManager:didUpdateToLocation:fromLocation:.
If we call delegate method after addAnnotation: it will be good, cause
it sets to nil after all usage.