apostrophe-schemas: Unable to edit page displayed in chooser withType apostrophe-page, although edit button exists

Every time I click "Edit" on a page item, a javascript error occurs. Node.js and Apostrophe are up to date. My module:

index.js
```js
module.exports = {
extend: 'apostrophe-widgets',
label: 'Menu',
addFields: [
{
name: '_pages',
type: 'joinByArray',
withType: 'apostrophe-page',
label: 'Page',
required: true,
filters: {
projection: {
title: 1,
slug: 1
}
}
}
]
};
```

widget.js
```nunjucks

{% for page in data.widget._pages %}

• 
  {{ page.title }}
  

{% endfor %}

```


Start with the joinByArray field in index.js and the page loop in widget.js, then reproduce the issue by selecting a page item and clicking Edit. Trace the resulting JavaScript error and verify that pages selected through the apostrophe-page chooser can be edited while the widget still renders their URLs and titles.

Written by the indexing model from the issue text.