Solving: Cast to ObjectId failed for value "" at path "_id". following freeCodeCamp.org's tutorial: React / GraphQL Course - Build a social media app (MERNG Stack) at 1:18:00

posts.js

/*
* When following freeCodeCamp.org's youtube video tutorial: React / GraphQL Course - Build a social media app (MERNG Stack)
* I have got stuck @1:18:00 because I got a weired error: Cast to ObjectId failed for value "" at path "_id".
* It turn out that that ModelName.findById() method only expect vaild mongodb ID, so all I did is
* to use Types.ObjectId.isValid from mongoose to check if the given string is vaild id or not, if not,
* Skipped making the query and I threw new "Post not found" Error.
*/

import mongoose from 'mongoose';

import { Post } from '../../Models/Post.js'

export const postResolvers = {
    Query: {
        getPosts: async () => {
            try {
                const posts = await Post.find();
                return posts;
            } catch (err) {
                throw new Error(err);
            }
        },
        getPost: async (_, { postId }) => {
            // validate if postId before is vaild ID before using it.
            const isValidId = mongoose.Types.ObjectId.isValid(postId)
            // if vaild
            if (isValidId) {
                try {
                    const post = await Post.findById(postId);
                    if(post) return post; 
                } catch (err) {
                    throw new Error(err);
                }
            } else {
                // if not valid, I'm skipping the query and I throwing new error with Post not found string. 
                throw new Error('Post not found');
            }
 
        }
    }
};