Firestore: Reading Document Issue - Document Can Not Be Found

For my Android App I am using Googles' Firebase.Firestore NOSQL database in order to store some data in a cloud DB.

I has no problem to save the data, but retrieving was not working.

So I reduced my test data structure to a minimum and tried all methods described in the documentation without success.

My - not working - minimal test data structure was:

This structure represents a collection of tests ("Tsts") of my App, containing one document for each version of my App, in this case version 11.
The document 11 contains a collection ("Devs") of devices for each device on with my app has been tested.

The solution of the problem was adding a field to the document 11, for example like this:

For testing how to read the data, I used the following code based on the Firestore documentation (Sub-chapter: Get a document).

DocumentReference docRef1 = mDB.collection("Tsts").document("11");

docRef1.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {

    @Override

    public void onComplete(@NonNull Task<DocumentSnapshot> task) {

        if (task.isSuccessful()) {

            DocumentSnapshot document = task.getResult();

            if (document.exists()) {

                mLog.debug("DocumentSnapshot data: {}", document.getData());

            } else {

                mLog.debug("No document");

            }

        } else {

            mLog.debug("get failed with ", task.getException());

        }

    }

});

Only after adding the test field "field1", I could read something.
Before adding field1, I always got the message "No document" in the logs.

I hope this info helps someone.

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