How to calculate the distance between two points with realtime firebase database, and also display the current distance he is travelling?

Question

I am trying to Develop a Vehicle Tracking app where I want to track Drivers activity Like DISTANCE, TIME TAKEN , AVRAGE SPEED. I am able to Track his current location and also see the path the Driver is going on in firebase database, Now I have found a code from a project for Distance, I have implemented it in my project.and the app is crashing, is there a different way to get and calculate distance, stopwatch, speed??

public void newJourney(){
    locations = new ArrayList<Location>();
}

public float getDistanceOfJourney(){
    if (locations.size() <=1){
        return 0;
    }

    return locations.get(0).distanceTo(locations.get(locations.size() - 1)) / 1000;

}


protected void onStart() {
    super.onStart();
    if (ContextCompat.checkSelfPermission(this,Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED)
    {
        startLocationUpdates();
    }
    initButtons();
    /////////NEW ///////////////////////
    new Thread(new Runnable() {
        @Override
        public void run() {
            float d = (float) getDuration();
            long duration = (long) d; // in seconds
            float distance = getDistanceOfJourney();

            long hours = duration / 3600;
            long minutes = (duration % 3600) / 60;
            long seconds = duration % 60;

            float avgSpeed = 0;
            if(d != 0){
             //   avgSpeed = distance / (d / 3600);
            }

            final String time = String.format("%02d:%02d:%02d", hours, minutes, seconds);
            final String dist = String.format("%.2f KM", distance);
            final String avgs = String.format("%.2f KM/H", avgSpeed);

            postBack.post(new Runnable() {
                @Override
                public void run() {
                    durationText.setText(time);
                    avgSpeedText.setText(avgs);
                   distanceText.setText(dist);
                }
            });

           try {
               Thread.sleep(500);

           } catch (Exception e){
               e.printStackTrace();;
           }
        }
    }).start();
}

after running this code am getting this error in log

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
    at com.example.vehicletracker.MapsActivity.getDistanceOfJourney(MapsActivity.java:116)
    at com.example.vehicletracker.MapsActivity$3.run(MapsActivity.java:293)
    at java.lang.Thread.run(Thread.java:923)

Below is the Code of tracking the current Activity and updating it in realtime:

 @Override
public void onLocationChanged(@NonNull Location location) {

    if (location != null) { //location is enabled
        saveLocation(location);

    } else { //When user has no access to location
        Toast.makeText(this, "No Location", Toast.LENGTH_LONG).show();
    }

    //polylines
    double latitude = location.getLatitude();
    double longitude = location.getLongitude();
    LatLng latLng = new LatLng(latitude, longitude);

    polypoints.add(latLng);
    redrawLine();

    //newUpdate
    if (recordLocations){
        locations.add(location);
    }
}

private void saveLocation(Location location) {
    databaseReference.setValue(location); //Saving Location of user To db
}