Odoo13 - odoo.sh

I have a custom model made via the UI, called x_custom_thing.

I want to extend that like so:

class XCustomThing(models.Model):
    _inherit = "x_custom_thing"
    
    company_id = fields.Many2one('res.company', 'Company', index=1)
    vendor_id = fields.Many2one('res.company', 'Vendor', index=1)

I get the error that

"TypeError: Model 'x_custom_thing' does not exist in registry".

How do I inherit and extend a custom model made via the UI? Can I?