Firebase Cloud Firestore : Invalid collection reference. Collection references must have an odd number of segments

Then you need change this:

db.collection("app/users/" + uid + "/notifications")...

for this:

db.collection("app").document("users").collection(uid).document("notifications")

You welcome ;)

Hierarchical data structures and subcollections are described in the documentation. A collection contains documents and a document may contain a subcollection. The structure is always an alternating pattern of collections and documents. The documentation contains this description of an example:

Notice the alternating pattern of collections and documents. Your collections and documents must always follow this pattern. You cannot reference a collection in a collection or a document in a document.

Thus, a valid path to a collection will always have an odd number of segments; a valid path to a document, an even number. Since your code is trying to query a collection, the path length of four is invalid.

You are missing collection reference. i.e db.collection(** This is getting null **).