Multivalue field in ContentEditor App fails with NPE - UI6

When trying to include a multivalve field within a content editor block it fails when you try to save the content.

Steps to reproduce

1. Go to demo author and open Resources app
2. Edit content-editor/blocks/text.yaml file
3. Include the snippet:
   content-editor/blocks/text.yaml

   [...] Rest of default block code
   assignedSegments:
     $type: jcrMultiValueField
     field:
       $type: comboBoxField
       datasource:
         $type: jcrDatasource
         workspace: website 

1. Save it and open the Stories app
2. Create a new story and include a Set block
3. On the new field included on step 3, select a page
4. Try to save the story
5. Check it fails

Expected results

The Story is saved successfully

Actual results

An exception is thrown (check attachments for full stack trace)

Dev notes

It only happens for the multiValue field, if you include another one, e.g, a multiField, it works:

[...] Rest of default block code
multi:
      i18n: true
      $type: jcrMultiField
      field:
        $type: compositeField
        properties:
          street:
            $type: textField
          city:
            i18n: true
            $type: textField