Error serializing `.menus.main[0].items[0].items` returned from `getStaticProps`

[taskrider2022]

Package containing the bug

next-drupal (NPM package)

Describe the bug

Reason: undefined cannot be serialized as JSON. Please use null or omit this value.

If you try to use in the next-drupal 2 beta to get the menu tree and add this to the getStaticProps return you will get this error.

I find out that the new class 'DrupalMenuTree' return a false empty value. It return 'undefined' instead of 'null'.

Expected behavior

A clear and concise description of what you expected to happen.

Steps to reproduce:

1. use the starter pages package
2. try to get menu in the getStaticProps
3. const { tree, items } = await drupal.getMenu('main');
4. then return them
5. return { props: { menus: { main: tree ?? null, }, resource, }, };
6. and you will get this
7. ⨯ N [Error]: Error serializing `.menus.main[0].items[0].items` returned from `getStaticProps` in "/[...slug]". Reason: `undefined` cannot be serialized as JSON. Please use `null` or omit this value.

Additional context

As workaround I create a copy of the 'DrupalMenuTree' class and only change one line.

import type { DrupalMenuItem, DrupalMenuItemId } from 'next-drupal';

export class DrupalMenuTree<
  T extends {
    id: DrupalMenuItemId;
    parent: DrupalMenuItemId;
    items?: T[];
  } = DrupalMenuItem,
> extends Array {
  parentId: DrupalMenuItemId;
  depth: number;

  constructor(menuItems: T[], parentId: DrupalMenuItemId = '', depth: number = 1) {
    super();

    this.parentId = parentId;
    this.depth = depth;

    if (menuItems?.length) {
      this.build(menuItems, parentId);
    }
  }

  build(menuItems: T[], parentId: DrupalMenuItemId) {
    // Find the children of the specified parent.
    const children = menuItems.filter((menuItem) => menuItem?.parent === parentId);

    // Add each child to this Array.
    for (const menuItem of children) {
      const subtree = new DrupalMenuTree<T>(menuItems, menuItem.id, this.depth + 1);

    // Change here the undefined to null
      this.push({
        ...menuItem,
        items: subtree.length ? subtree : null,
      });
    }
  }
}

After that I will get the menu and took the items and build a tree with the new class.

return {
    props: {
      menus: {
        main: menu ?? null,
      },
      resource,
    },
  };

And it will work