How to get nested push key from Firebase database Android

I have my DB structure like this:

When someone follows someone else or likes a picture or something, the notification goes into the Notifications table followed by the users id(who sent the notif) followed by a unique id to separate the notifications. If I don't add that unique id(push()) only one notification is added and when a new one is supposed to be added only the time and text of the notification changes.

This is how I add the notification:

private void addNotifications(String userid, String postid) {

    DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Notifications").child(userid);

    String notificationid = reference.getKey();

    mGroupId = reference.push().getKey();

    HashMap<String, Object> hashMap = new HashMap<>();
    hashMap.put("dateAdded", System.currentTimeMillis());
    hashMap.put("date_time", getTimeDate());
    hashMap.put("notificationid", notificationid);
    hashMap.put("userid", firebaseUser.getUid());
    hashMap.put("text", "liked your post");
    hashMap.put("postid", postid);
    hashMap.put("ispost", true);

    reference.child(mGroupId).setValue(hashMap);
    Log.d("Post Key", mGroupId);
}

This shows me in the logcat the correct push key that was just added Log.d("Post Key", mGroupId);.

I am trying to delete anything older than a day in my notifications table. When I try to access mGroupId in the notifications fragment I get this error java.lang.NullPointerException: Can't pass null for argument 'pathString' in child().

I have been checking answers and comments for days now from experts like Frank Van P. and Alex Mamo and from regular people. I tried downvoted answers and comments and still nothing. I asked this before but the question keeps getting closed as a duplicate even though none of the duplicates help. How can I proceed?

How I try to delete the notifications older than a day:

private void lessThanADayNotification() {

    DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Notifications").child(firebaseUser.getUid());
    DatabaseReference freezerItemsRef = reference.child(PostAdapter.mGroupId);
    //Log.i("uid", uid);

    long cutoff = new Date().getTime() - TimeUnit.MILLISECONDS.convert(12, TimeUnit.SECONDS);
    Query oldBug = reference.child("Notifications").orderByChild("dateAdded").endAt(cutoff);
    oldBug.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot snapshot) {

            Notification notification = new Notification();
            freezerItemsRef.child(notification.getNotificationid()).removeValue();
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });
}

I've said this before on the question that you since deleted: there is no way to query on your dateAdded property from the Notifications node.

Firebase queries operate on a flat list of the child nodes directly on the node where you query. So in your data model you can query notifications for a specific user, but you can't query notifications across all users.

If you want to query across the notifications of all users, you will need to store them as a flat list, with the UID in that case stored as a property of each notification node.

See:

• https://stackoverflow.com/questions/27207059/firebase-query-double-nested
• https://stackoverflow.com/questions/40656589/firebase-query-if-child-of-child-contains-a-value