IDAlreadyInUseError when creating document and subscribing to onChildCreated event

[Frost-bit]

In the same file I create a new document inside a container called notes:

createNote(title, content, topics) {
	let newNote = {
	    title: title,
	    content: content,
	    hasMemberRelation: "topics",
	    isMemberOfRelation: "notes"
        };
        return this.notes.$create(newNote)
	    .then(() => {
		//add members to the new document
		//...
	    })

...and subscribe to the event of the creation of new notes with:

subscribe() {
	this.notes.$onChildCreated(sendEmail);
    }

I am getting the error IDAlreadyInUseError at Object._addPointer (carbonldp/Registry/Registry.js:110:23), even if without the subscription, the code for creating the document works perfectly.