Swift - Generate an Address Format from Reverse Geocoding

I am trying to generate a Formatted Full address using CLGeocoder in Swift 3. I referred to this SO thread to get the code given below.

However, sometimes the app crashes with a 'nil' error at the line:

//Address dictionary
print(placeMark.addressDictionary ?? "")

Questions:

1. How can I concatenate these values retrieved from the GeoCoder to form a full address? (Street + City + etc)
2. How do I handle the nil error I get when the func is unable to find an address?

Full code:

func getAddress() -> String {
        var address: String = ""

        let geoCoder = CLGeocoder()
        let location = CLLocation(latitude: selectedLat, longitude: selectedLon)
        //selectedLat and selectedLon are double values set by the app in a previous process

        geoCoder.reverseGeocodeLocation(location, completionHandler: { (placemarks, error) -> Void in

            // Place details
            var placeMark: CLPlacemark!
            placeMark = placemarks?[0]

            // Address dictionary
            //print(placeMark.addressDictionary ?? "")

            // Location name
            if let locationName = placeMark.addressDictionary!["Name"] as? NSString {
                //print(locationName)
            }

            // Street address
            if let street = placeMark.addressDictionary!["Thoroughfare"] as? NSString {
                //print(street)
            }

            // City
            if let city = placeMark.addressDictionary!["City"] as? NSString {
                //print(city)
            }

            // Zip code
            if let zip = placeMark.addressDictionary!["ZIP"] as? NSString {
                //print(zip)
            }

            // Country
            if let country = placeMark.addressDictionary!["Country"] as? NSString {
                //print(country)
            }

        })

        return address;
    } 

func getAddressFromLatLon(pdblLatitude: String, withLongitude pdblLongitude: String) {
        var center : CLLocationCoordinate2D = CLLocationCoordinate2D()
        let lat: Double = Double("\(pdblLatitude)")!
        //21.228124
        let lon: Double = Double("\(pdblLongitude)")!
        //72.833770
        let ceo: CLGeocoder = CLGeocoder()
        center.latitude = lat
        center.longitude = lon

        let loc: CLLocation = CLLocation(latitude:center.latitude, longitude: center.longitude)


        ceo.reverseGeocodeLocation(loc, completionHandler:
            {(placemarks, error) in
                if (error != nil)
                {
                    print("reverse geodcode fail: \(error!.localizedDescription)")
                }
                let pm = placemarks! as [CLPlacemark]

                if pm.count > 0 {
                    let pm = placemarks![0]
                    print(pm.country)
                    print(pm.locality)
                    print(pm.subLocality)
                    print(pm.thoroughfare)
                    print(pm.postalCode)
                    print(pm.subThoroughfare)
                    var addressString : String = ""
                    if pm.subLocality != nil {
                        addressString = addressString + pm.subLocality! + ", "
                    }
                    if pm.thoroughfare != nil {
                        addressString = addressString + pm.thoroughfare! + ", "
                    }
                    if pm.locality != nil {
                        addressString = addressString + pm.locality! + ", "
                    }
                    if pm.country != nil {
                        addressString = addressString + pm.country! + ", "
                    }
                    if pm.postalCode != nil {
                        addressString = addressString + pm.postalCode! + " "
                    }


                    print(addressString)
              }
        })

    }