Wagtail link block requiring exactly one of 3 options

blocks.py

class AirspaceRequiredLinkBlock(blocks.StructBlock):
    """
    Allows a user to create a link to a Page, a Document, or a relative or absolute URL.
    This version requires that you provide one and only one of the destinations above.

    NOTE: Due to limitations in CSS, callers of LinkBlock() must not specify a label in the construction arguments.
    See the comment in the Meta class for why.

    NOTE: Within a template, checking for the existence of `self.link` will always return True because the LinkBlock
    object is not falsy, even if it has no contents. To retrieve the value of a LinkBlock, use the {% link_url %}
    template tag from airspace_tags. ex:
        {% load airspace_tags %}
        {% link_url self.link as url %}
        {% if url %}
            <a href={{ url }}></a>
        {% endif %}
    """
    page = blocks.PageChooserBlock(
        required=False,
        help_text="Link to the chosen page. If you are going to a page on this site, use this option if possible. "
                  "NOTE: you must choose one and only one of 'page', 'document' or 'url'."

    )
    document = DocumentChooserBlock(
        required=False,
        help_text="Link to the chosen document."
    )
    url = blocks.CharBlock(
        required=False,
        help_text="Link to the given URL. This can be a relative URL to a location your own site (e.g. /example#FAQ1) "
                  "or an absolute URL to a page on another site (e.g. http://www.caltech.edu). Note: absolute URLs "
                  "must include the http:// otherwise they will not work."
    )

    class Meta:
        label = 'Link'
        form_classname = 'link-block'

    def clean(self, value):
        result = []  # build up a list of (name, value) tuples to be passed to the StructValue constructor
        errors = {}
        found_items = []
        for name, val in value.items():
            try:
                result.append((name, self.child_blocks[name].clean(val)))
                # Blank values are None for 'page' and 'document' but blank urls are '', so check length of str(val)
                # We need to str() the value because if there is a document or page, the classes don't have len() method
                if val and len(str(val)) > 0:
                    found_items.append(val)
            except ValidationError as e:
                errors[name] = ErrorList([e])

        if not found_items:
            errors['page'] = ErrorList(["You must provide one of 'page', 'document' or 'url'"])
        if len(found_items) > 1:
            first_item = result[0][0]
            errors[first_item] = ErrorList(["You may provide only one of 'page', 'document' or 'url'"])

        if errors:
            # The message here is arbitrary - StructBlock.render_form will suppress it
            # and delegate the errors contained in the 'params' dict to the child blocks instead
            raise ValidationError('Validation error in StructBlock', params=errors)

        return self._to_struct_value(result)