Help getting firestore-information

I know it is a very specific problem, but I am trying figuring this out for 2 days now – and I can’t. Maybe somebody can help. It’s about creating a shortcode, that gives information from one specific firestore-field back. But here the details:

My Firestore:

Links-coll.
|
*-- link_id [String]
|
*-- object_ref [Reference to Objekte-collection-document]

Objekte-coll.
|
*-- name [String]

My aim:
1. Find the document in Links, where the field “link_id” matches a given string
2. Show the name of the object from the Objekte-collection to which the “objekt_ref”-field in the Links-collection points to.

What I came up with so far:

function getObjectIdFromLinkId(linkId) {
	const showFirestoreDatabase = () => {
    	const db = firebase.firestore();

		db.collection("Links")
			.where("link_id", "==", linkId)
    		.get()
    		.then((querySnapshot) => {
        		querySnapshot.forEach((doc) => { 
					// there should be only one
					// in the field "object_ref" is the reference to another collection/document
					var docRef = db.doc(doc.data().object_ref);
					docRef.get().then((doc) => {
    					if (doc.exists) {
        					console.log("Document data:", doc.data());
    					} else {
        					console.log("No such document!");
    					}
					})
					.catch((error) => {
    					console.log("Error getting Object-document:", error);
					});
        		});
    		})
    		.catch((error) => {
        		console.log("Error getting Link-documents: ", error);
    		});
	}

	const waitForFirebase = setInterval(() => {
	if (typeof firebase !== 'undefined' && firebase.apps.length) {
        	showFirestoreDatabase()
        	clearInterval(waitForFirebase)
    	}
	}, 1000)
}

Like this I get the error “TypeError: n.indexOf is not a function”

I would be really happy if somebody could help.
Thanks a lot in advance!