I want to add data to an ArrayList inside a For-Loop and it doesn't work.

I've got a Cloud Firestore in Firebase with a collection filled with documents by another app. This one is meant to get the data and put it in a Recycler View. Adapter and all are set. Far as I can see, the problem is with filling the (Array)list inside the For-Loop? (As also just filling in any data inside this loop doesn't work, but I'm not sure.)
I noted some of my tries and errors in the code using comments. I'm sure, it must be just some minor mistake, but I really can't find it.
Adding data with the exact same code outside the For-Loop works, inside doesn't work (see "Blah")

```
private ArrayList<String> patients;

private String testString;
//-> String einzig zu Testzwecken

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

    patients = new ArrayList<>();

    textView = findViewById(R.id.textView);

    db.collection("Test")
            .get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {
                        for (QueryDocumentSnapshot document : task.getResult()) {

                            //patients = new ArrayList<>();
                            //-> doesn't work

                            patients.add((document.getData().toString()));
                            //doesn't work

                            listData.add(new Data(R.drawable.hohlbrot, "ahjotest"));
                            //doesn't work (trying to feed the recyclerView more directly)

                            //Toast.makeText(MainActivity.this, document.getData().toString(), Toast.LENGTH_SHORT).show();
                            //textView.setText(document.getData().toString());

                                // -> both work
                            testString = testString + document.getData().toString();
                            textView.setText(testString);
                                // -> works
                            patients.add("Blah1");
                            //doesn't work

                            //Log.d(TAG, document.getId() + " => " + document.getData());
                        }
                    } else {
                        //Log.d(TAG, "Error getting documents: ", task.getException());
                        patients.add("BlahError");
                    }
                }
            });

    patients.add("Blah2");
      //works

```

Well, I tried several things, but with the setting, that I would expect to work, there were no error messages. It just didn't do anything. In the RecyclerView the "Blah2" occurs as do any Strings I add later. Anything I tried inside the For-Loop (Toast, String-to-TextView,...) actually worked, as long as it didn't use the ArrayList.

PS: Me am Noob, beg your patience -.-