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")...
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 **).