Wagtail ModelAdmin not showing fields when editing one object, but does show fields when showing all

Question

I'm using Wagtail, and I'm also using django-inspectional-registration. I'm trying to set it up so that approving users can be done in the CMS, and not the django admin. To do this I've created a ModelAdmin, as seen here:

wagtail_hooks.py

from wagtail.contrib.modeladmin.options import (ModelAdmin, modeladmin_register)
from registration.models import RegistrationProfile

class RegUserModelAdmin(ModelAdmin):
    model = RegistrationProfile
    menu_label = 'Users to accept'
    menu_icon = 'user'
    menu_order = 250
    add_to_settings_menu = False
    exclude_from_explorer = False
    list_display = ("user", "_status")
    list_filter = ("_status",)
    search_fields = ("user",)

modeladmin_register(RegUserModelAdmin)

Then as expected all registration profiles are seen like so:

However if I click on any of these profiles, it shows me nothing:

No errors appear in the browser console, or my test server, absolutely nothing.

I have another ModelAdmin that works perfectly with another model of mine, which makes this even more confusing.