"Decorated type deeper than introspection query" error with nested lists

从嵌套的 Query.animals schema 及其 resolve_animals 示例开始，然后通过 GraphiQL 0.7.1 重现失败，同时检查 introspection 响应。将嵌套的 NonNull/list 情况与移除 NonNull 的版本进行比较；当 schema 能够进行 introspection 且 GraphiQL 不再报告 decorated-type 错误时，即视为完成。

We see the following errors when using nested lists:

Error: Decorated type deeper than introspection query.
    at n (http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:15:23637)
    at n (http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:15:23879)
    at n (http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:15:23733)
    at n (http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:15:23879)
    at r (http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:15:24425)
    at http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:15:26609
    at http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:11:8480
    at Array.reduce (native)
    at keyValMap (http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:11:8444)
    at f (http://cdn.jsdelivr.net/graphiql/0.7.1/graphiql.min.js:15:26484)

Here's the example code that can cause this issue. Graphene introspection doesn't report any errors.

import graphene, json

class Animal(graphene.Interface):
   id = graphene.String()

class Dog(graphene.ObjectType):
   class Meta:
      interfaces = (Animal, )
   breed = graphene.String()

class Cat(graphene.ObjectType):
   class Meta:
      interfaces = (Animal, )
   category = graphene.String()

class Query(graphene.ObjectType):
  animals = graphene.NonNull(graphene.List(graphene.NonNull(graphene.List(graphene.NonNull(Animal)))))
  def resolve_animals(self, args, context, info):
    return[[Dog(id=1, breed='Golden Retriever'),
            Dog(id=2, breed='German Shepard')],
           [Cat(id=11, category ='Tiger'),
            Cat(id=12, category='House Cat')]]

schema = graphene.Schema(query=Query, types=[Dog, Cat, ])

Errors disappear if we remove the "required" or NonNull decorators. Any suggestions on how to fix these errors?