I am trying to get the list of documents from a collection but having the out-of-range error. You can see the function that returns the list and the output afterward.

Future<List> getPreAppliedUserList() async {
    try {
      List? userList = [""];
      QuerySnapshot querySnapshot =
          await _db!.collection(_preAppliedUserCollection).get();
      List allData = querySnapshot.docs.map((doc) => doc.data()).toList();
      print(allData.length); /// 2
      for (int i = 0; i <= allData.length; i++) {
        userList[i] = await allData[i]['customerNumber'];
      }
      return userList;
    } on FirebaseException {
      print("getPreAppliedUserList error");
      var x; ///todo
      return x;
    }
  }

Error:

Error: RangeError (index): Index out of range: index should be less than 1: 1