How I get ExoPlayer to resume a video in my Activity's onStart() method?

I'm using ExoPlayer to play some mp4's from a URL. When the user click's the home button, or anything that causes the app to be removed from the user's view and then comes back to my app (and video activity) I want the video to resume where it left off. I tried implementing this by saving the video position in onStop() and then rebuilding the player and using seekTo() in onStart(). I have a check to see if my current exoplayer is null in onStart(), this check never passes however, so I think this is where the problem lies. How it's coded now my video never resumes. If I leave the app by pressing the home button onStop() gets called and then onStart() will get called when I go back into my app, however the video player remains black and never plays the video. If I remove the null check I get two instances of the video playing whenever I start a video from the main activity because it gets called both in onCreate() and onStart(). Is there a better method for getting the functionality that I want? Any help is appreciated!

public class VideoActivity extends AppCompatActivity {

    private SimpleExoPlayer exoPlayer;
    private SimpleExoPlayerView simpleExoPlayerView;
    private long playerPosition;
    private String mp4Url;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.player_activity);

        // Get the Intent that started this activity and extract the video url
        Intent intent = getIntent();
        mp4Url = intent.getStringExtra(MainActivity.VIDEO_URL);
        // Create an exoplayer instance and start playing video
        buildPlayer(mp4Url);

    }

    private void buildPlayer(String mp4Url) {
        // Create a default TrackSelector
        Handler mainHandler = new Handler();
        BandwidthMeter bandwidthMeter = new DefaultBandwidthMeter();
        TrackSelection.Factory videoTrackSelectionFactory = new AdaptiveTrackSelection.Factory(bandwidthMeter);
        TrackSelector trackSelector = new DefaultTrackSelector(videoTrackSelectionFactory);

        // Create the player
        exoPlayer = ExoPlayerFactory.newSimpleInstance(this, trackSelector); // no LoadControl?
        simpleExoPlayerView = new SimpleExoPlayerView(this);
        simpleExoPlayerView = (SimpleExoPlayerView) findViewById(R.id.player_view);

        // Set media controller
        simpleExoPlayerView.setUseController(true);
        simpleExoPlayerView.requestFocus();

        // Bind player to the view
        simpleExoPlayerView.setPlayer(exoPlayer);

        // Create Uri from video location
        // TODO: should this be in some network class? Should I be appending APIKEY here?
        Uri mp4Uri = Uri.parse(mp4Url + "?api_key=" + BuildConfig.GIANTBOMB_API_KEY);
        Timber.v("Video url with api key: " + mp4Uri.toString());

        // Create another bandwidth meter for bandwidth during playback (not strictly necessary)
        DefaultBandwidthMeter playbackBandwidthMeter = new DefaultBandwidthMeter();

        // DataSourceFactory to produce DataSource instances through which media data is loaded
        DefaultDataSourceFactory dataSourceFactory = new DefaultDataSourceFactory(this,
                Util.getUserAgent(this, "GiantBombForAndroid"),
                playbackBandwidthMeter);

        // Produces Extractor instances for parsing the media data
        ExtractorsFactory extractorsFactory = new DefaultExtractorsFactory();

        ExtractorMediaSource.EventListener eventListener = new ExtractorMediaSource.EventListener() {
            @Override
            public void onLoadError(IOException error) {
                Timber.e("Error loading video from source");
            }
        };

        final MediaSource videoSource = new ExtractorMediaSource(mp4Uri,
                dataSourceFactory,
                extractorsFactory,
                mainHandler,
                eventListener);

        exoPlayer.prepare(videoSource);

        exoPlayer.addListener(new ExoPlayer.EventListener() {
            @Override
            public void onLoadingChanged(boolean isLoading) {
                Timber.v("Listener-onLoadingChanged...");

            }

            @Override
            public void onPlayerStateChanged(boolean playWhenReady, int playbackState) {
                Timber.v("Listener-onPlayerStateChanged...");

            }

            @Override
            public void onTimelineChanged(Timeline timeline, Object manifest) {
                Timber.v("Listener-onTimelineChanged...");

            }

            @Override
            public void onTracksChanged(TrackGroupArray trackGroups, TrackSelectionArray trackSelections) {
                // TODO: Do I need anything here?
            }

            @Override
            public void onPlayerError(ExoPlaybackException error) {
                Timber.v("Listener-onPlayerError...");
                exoPlayer.stop();
                exoPlayer.prepare(videoSource);
                exoPlayer.setPlayWhenReady(true);
            }

            @Override
            public void onPositionDiscontinuity() {
                Timber.v("Listener-onPositionDiscontinuity...");

            }

            @Override
            public void onPlaybackParametersChanged(PlaybackParameters playbackParameters) {
                // TODO: Do I need anything here?
            }
        });
        exoPlayer.setPlayWhenReady(true);
    }

    @Override
    protected void onStart() {
        super.onStart();
        Timber.v("onStart()...");
        if (exoPlayer == null) {
            Timber.v("No exoplayer instance, recreating...");
            buildPlayer(mp4Url);
            exoPlayer.seekTo(playerPosition);
        }
    }

    @Override
    protected void onStop(){
        super.onStop();
        Timber.v("onStop()...");
        //TODO: pull player creation code into it's own method so it can be called here as well
        playerPosition = exoPlayer.getCurrentPosition();
        exoPlayer.release();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Timber.v("onDestroy()...");
        exoPlayer.release();
    }
}