title: ApostropheCMS error during editor modal save when joining a piece to a module
body: <p>I created a new "modal" piece where I'm joining another module (so I can display modules inside the modal). I can select the module in the editor modal, but when I save I get this error pop up: <code>An error occurred. Please try again later.</code> Front-end console shows this is coming from <code>/modules/apostrophe-browser-utils/js/always.js</code></p>
  
  <p>All I have is the <strong>/lib/modules/modal/index.js</strong> file like this:</p>
  
  <pre><code>module.exports = {
      extend: "apostrophe-pieces",
      name: "modal",
      label: "Modal",
      seo: false,
      addFields: [
        {
          name: "_image",
          type: "joinByOne",
          withType: "apostrophe-image",
          label: "Pick an image",
          required: true,
          idField: "_id",
          filters: {
            // Thought maybe not having a projection was an issue. didn't help
            projection: {
              attachment: 1
            }
          }
        }
      ]
    };
  </code></pre>
  
  <p>Screen Cap: <a href="https://screencast.com/t/OGQOD0PiG" rel="nofollow noreferrer">https://screencast.com/t/OGQOD0PiG</a></p>