title: Laravel Cashier: Cancelling a subscription gives error "Call to a member function asStripeCustomer() on null"
body: <p>I am using Laravel Cashier for two types of subscriptions - for companies and individual users. I added the Billable trait to the Company model and I created a PersonalBillable trait for the User model. I also have a PersonalSubscription class where I specify that the User subscriptions should use 'personal_subscriptions' table. Company subscriptions use the default 'subscriptions' table that comes with Cashier migrations. Everything works perfectly fine except when I try to cancel a User subscription. The following code 
  <code>auth()-&gt;user()-&gt;subscription('user')-&gt;cancel();</code> gives an error "Call to a member function asStripeCustomer() on null". 
  Does anyone know what might be causing this?</p>