Yjs/community

In the present arrangement of my app, I have two yMaps. One (yMapA) stores objects keyed by their ids. The other (yMapB) stores metadata about the objects, including references to their ids, e.g. which of the objects is currently 'selected' by the user. This all works fine, except when a new client arrives and syncs with the others (using a WebSocketProvider). Mostly, yMapB loads first and then code in yMapB.observe tries to select the referenced objects in yMapA. However, yMapA has not yet been loaded and so this operation fails. Schematically, it looks like this (this is a minimal example simplified from the actual code):

const doc = new Y.Doc();
const wsProvider = new WebsocketProvider(websocket, room, doc);
const yMapA = doc.getMap('A');
const yMapB = doc.getMap('B');
wsProvider.on('sync', () => {
        // do something
    }); 
yMapA.observe((event) => {
    for (let key of event.keysChanged) {
        if (yMapA.has(key)) {
            let obj = yMapA.get(key)
            // add or update local copy  of obj
        }
        else {
            // delete local copy of obj
        }
})
yMapB.observe((event) => {
    for (let key of event.keysChanged) {
        let selection = yMapB.get(key);
        selection.forEach((id) => {
            // find object with id
            // mark it as selected
        }
    }
})

How should I rewrite this to avoid the problem?