Fix the bug where trying to unsubscribe a customer with no subscription throws an error.

endpoints/api.py

@@ -1217,8 +1217,10 @@ def subscribe(user, plan, token, accepted_plans):
     cus = stripe.Customer.retrieve(user.stripe_id)
 
     if plan_found['price'] == 0:
-      cus.cancel_subscription()
-      cus.save()
+      if cus.subscription is not None:
+        # We only have to cancel the subscription if they actually have one
+        cus.cancel_subscription()
+        cus.save()
 
       response_json = {
         'plan': plan,