LateError (LateInitializationError: Field 'documentSnapshot' has not been initialized)

Question

I am using Cloud Firestore in my Flutter application and I am facing this issue in app: LateError (LateInitializationError: Field 'documentSnapshot' has not been initialized). I am using documentSnapshot in app but I don't know what should be used in its initialization as it takes a field value from Cloud Firestore.

I have declared a late documentSnapshot. and here I am reading the data from database using that documentSnapshot. It is throwing an error as it is a late variable and I am accessing it before initializing.

This is the code :

Future<int> getBudget(userKey) async {
  
  var check = await FirebaseFirestore.instance
      .collection(userKey!.email!)
      .doc('limit')
      .get();
  if (check.exists) {
    FirebaseFirestore.instance
        .collection(userKey!.email!)
        .doc('limit')
        .get()
        .then((value) {
      documentSnapshot = value;
      setState(() {
        _budget = documentSnapshot['limit'];
      });
    });
    return documentSnapshot['limit'];
  }
  return 0;
}