No bidirectional relations when one of the contenttypes is a singleton.

[evertalbers]

I notice that Bolt errors when trying to make relations between a singleton and a normal contenttype.

You should be able to reproduce this with the shortened contenttypes below. (You don't need a working frontend as this bug happens in the backend).

The code below works, to see the error set the homepage to singleton: true, open an edition and make a relation to the homepage and you should see it.
I did not see a difference between multiple: true and multiple: false.

Tested on php7.x & php8.0, Bolt5.1.18 & 5.1.20

homepages:
    name: Homepages
    singular_name: Homepage
    fields:
        title:
            type: text
            class: large
            group: content
            label: Title
        image:
            type: image
        introduction:
            type: redactor
        content:
            type: redactor
        slug:
            type: slug
            uses: title
            group: Meta
            prefix: The homepage with slug 'homepage' will be used as actual homepage.
    viewless: false
    relations: 
        editions:
            label: 'Select the edition(s) to be shown on the homepage'
            # postfix: It is not recommended to show more than 1 but it works, technically
            multiple: true
            required: false
    locales: ['en']
    singleton: false
    icon_many: "fa:home"
    icon_one: "fa:home"

editions:
    name: Editions
    singular_name: Edition
    fields:
        title:
            type: text
            class: large
        slug:
            type: slug
            uses: title
            group: Meta
    relations: 
        speakers:
            label: Select related speakers
            multiple: true
            required: false
        homepages:
            label: Select related homepage
            # prefix: This will make this item appear on top of the homepage
            multiple: false
            required: false
            order: -id
    records_per_page: 50
    order: id

speakers:
    name: Speakers
    singular_name: Speaker
    title_format: "{name} ({description})"
    fields:
        name:
            type: text
            class: large
            variant: inline
            fixture_format: '{name}'
        slug:
            type: slug
            uses: [ name ]
    relations: 
        editions:
            label: Select related editions
            multiple: true
            required: false
    icon_one: fa:user
    icon_many: fa:users

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