ManyToMany Repeater: Call to a member function subscriptionProducts() on null

I have followed this guide: https://filamentphp.com/docs/3.x/forms/fields/repeater#integrating-with-a-belongstomany-eloquent-relationship but unfortunately no luck.

My models:

subscription:
- id
- name

productSubscription
- id
- product_id
- subscription_id
- price
- quantity

product:
- id
- name
- tax
- default_price

ManyToMany Repeater: Call to a member function subscriptionProducts() on null - Filament