Cache is empty after the Activity is returned

I am having this issue on an app that I made. It's a simple game that I loads data from Room database in my onCreate method, saves the loaded data in List<GameData> gameData and then starts the game.

gameDao.getAll()
    .subscribe(gameDatas -> {
        gameData.addAll(gameData);
        startGame();
    });

inside my startGame, I randomly select a number in the range [0, gameData.size()] and do some game related work with the corresponding gameData.

public void startGame(){
    Random random = new Random();
    int randomNumber = random.nextInt(gameData.size());

    // do something with gameData[ramdomNumber]
}

So, then user keeps playing and then when the user loses I send him to GameOverActivity with startActivityForResult(gameOver,1123)

Inside GameOverActivity user can

• restart the game
• watch an ad to continue playing
• other social related staff

So when the user presses restart button, I just finish the GameOverActivity which will redirect me to onActivityResult of GameActivity. Here I just call startGame method again, and it should supposedly start a new game.

Now the problem is sometimes for certain devices, gameData is getting empty and hence I am getting exception inside startGame when I call random.nextInt(gameData.size())

java.lang.RuntimeException: 

  at android.app.ActivityThread.performResumeActivity (ActivityThread.java:3790)

  at android.app.ActivityThread.handleResumeActivity (ActivityThread.java:3830)

  at android.app.ActivityThread.handleLaunchActivity (ActivityThread.java:3038)

  at android.app.ActivityThread.-wrap11 (Unknown Source)

  at android.app.ActivityThread$H.handleMessage (ActivityThread.java:1696)

  at android.os.Handler.dispatchMessage (Handler.java:105)

  at android.os.Looper.loop (Looper.java:164)

  at android.app.ActivityThread.main (ActivityThread.java:6944)

  at java.lang.reflect.Method.invoke (Native Method)

  at com.android.internal.os.Zygote$MethodAndArgsCaller.run (Zygote.java:327)

  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1374)
Caused by: java.lang.RuntimeException: 

  at android.app.ActivityThread.deliverResults (ActivityThread.java:4491)

  at android.app.ActivityThread.performResumeActivity (ActivityThread.java:3762)
Caused by: java.lang.IllegalArgumentException: 

  at java.util.Random.nextInt (Random.java:388)

  at com.company.game.GameActivity.startGame (GameActivity.java:149)

  at com.company.game.GameActivity.onActivityResult (GameActivity.java:477)

  at android.app.Activity.dispatchActivityResult (Activity.java:7556)

  at android.app.ActivityThread.deliverResults (ActivityThread.java:4487)

I am not understating why would gameData is getting cleared, and I am not able to check because I have never encountered while testing and I got the crash report from PlayConsole. Any idea what might be causing this?