Storing Values From Firestore Cloud Database Into an Arraylist in Java (Android)

Question

So am trying to retrieve values from firestore database, then store them into an Arraylist.There after am trying to choose the values at random from the array, then am updating a different field in the document ("GameState") while using the randomly selected value as a Key to update the ("GameState") field in the document. This what I have tried: But when I run it, the app crashes.

GamePortals.get()
    .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull @NotNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()){
            ArrayList<Integer> numberPot;
                        numberPot = new ArrayList<>();
                        int randomPicker = (int) (Math.random()*4);

                    for (QueryDocumentSnapshot document: task.getResult()){
                        //displayData.setText(String.valueOf(document.get("GameCode")));

                        
                        numberPot.add((Integer) document.get("GameCode"));
                        numberPot.get(randomPicker);

                        //displayData.setText(String.valueOf(numberPot));          
                        //displayData.setText(numberPot.get(randomPicker));
                       // Log.d(TAG,"onCreate: numbers:" + numberPot.get(randomPicker));

                        GamePortals.whereEqualTo("GameCode", numberPot.get(randomPicker))
                                .get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                            @Override
                            public void onComplete(@NonNull @NotNull Task<QuerySnapshot> task) {
                                if (task.isSuccessful()){
                                    document.getReference().update("GameState","Winner");
                                    Toast.makeText(getApplicationContext(),"Updating was successful!", Toast.LENGTH_SHORT).show();
                                }else
                                {
                                    Toast.makeText(getApplicationContext(),"Updating was not successful!", Toast.LENGTH_SHORT).show();
                                }
                            }
                        });
                    }

            displayData.setText(String.valueOf(numberPot.get(randomPicker)));

                }else{
                    Toast.makeText(getApplicationContext(),"Some is wrong with Query", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }

Clould Firestore Database

ERROR LOGCAT filtered by Error logcat image

LINE 83 (CODE SCREENSHOT ) Contains screenshot of the code(referenced in error logcat)