title: Ambiguous use of observe firebase DB
body: <p>I really cannot get what is wrong?<br>
  I'm trying to load some settings data from firebase <code>Settings</code> node. Same code for other nodes in other functions work but this one is ambiguous. Why?</p>
  
  <pre><code>var ref:FIRDatabaseReference!  //Global variable
  
       override func viewDidLoad() {
          super.viewDidLoad()
  
  
          self.mapView.delegate = self
  
              if CLLocationManager.locationServicesEnabled() {
  
                  locationManager.delegate = self
                  locationManager.desiredAccuracy = kCLLocationAccuracyBest
                  locationManager.requestWhenInUseAuthorization()
                  locationManager.startUpdatingLocation()
  
                //  monitorRegion()
  
              } else {
  
                  // Alert to enable location services on iphone first
              }
  
              ref = FIRDatabase.database().reference(fromURL: "https://*******.firebaseio.com/")
              //The error is here
              ref.child("Settings").child("service_types").observe(.value) { (snapshot) in
  
              }
  
  
          // Do any additional setup after loading the view.
      }
  </code></pre>