solution.dart

Future getPosts(BuildContext context) async {
  int index = getFromSF();
  QuerySnapshot querySnapshot =
      await Firestore.instance.collection('fortunepool').getDocuments();

if (indexList.contains(index)) {
    // TODO: get any other document
    int currentIndex = index;
    String myFortune = "";


    // This should return you the next title
     while(index < querySnapshot.documentChanges.length && indexList.contains(currentIndex)){
      myFortune = querySnapshot.documents[index++].data['title'];
      currentIndex = index++;
    }
      myFortune = querySnapshot.documents[index++].data['title'];
      currentIndex = index++;
    }
  
    saveDocumentInSharedPreferences(myFortune);
    setIndexToSharedFortune(index);
    indexList.add(index);
    saveIndexNumbers(indexList, "listofIndexes");
  } else if (indexList.isEmpty || index == 5) {
  // Keep this part if is working
  }