Bug report for https://github.com/kevindew/openapi3_parser

bug.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'openapi3_parser', '= 0.9.2'
end

require 'pathname'
require 'yaml'
require 'openapi3_parser'

raw = YAML.load_file('openapi.yaml')

puts 'Parsing the YAML works fine:'
doc = Openapi3Parser.load(raw)
puts '  OpenAPI document parsed successfully'
puts

puts 'I can access each component directly and everything is fine:'
doc.components.schemas.keys.each do |key|
  puts "  #{key} => #{doc.components.schemas[key].inspect}"
end
puts

puts 'If I access a component that uses components/schemas/Null though via a $ref, I get an error:'
puts

doc.paths['/blog/post'].post.responses['201'].content['application/json'].schema

openapi.yaml

---
openapi: 3.0.3

info:
  title: Bug Report
  version: v0

paths:
  /blog/post:
    post:
      description: x
      responses:
        "201":
          description: x
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/BlogPost"
components:
  schemas:
    "Null":
      not:
        anyOf:
          - type: string
          - type: number
          - type: boolean
          - type: object
          - type: array
      description: |-
        The literal value `null`.

        NOTE: This is a workaround for OpenAPI 3.0's lack of support for nullable ref types. See: https://github.com/OAI/OpenAPI-Specification/issues/1368#issuecomment-580103688
      title: "Null"
    
    Description:
      type: string

    BlogPost:
      type: object
      properties:
        title:
          type: string
        
        body:
          type: string

        description:
          oneOf:
            - "$ref": "#/components/schemas/Description"
            - "$ref": "#/components/schemas/Null"

output.txt

$ ruby bug.rb

Parsing the YAML works fine:
  OpenAPI document parsed successfully

I can access each component directly and everything is fine:
  Null => Openapi3Parser::Node::Schema(#/components/schemas/Null)
  Description => Openapi3Parser::Node::Schema(#/components/schemas/Description)
  BlogPost => Openapi3Parser::Node::Schema(#/components/schemas/BlogPost)

If I access a component that uses components/schemas/Null though via a $ref, I get an error:

/Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node_factory/field.rb:110:in `validate': Invalid data for #/paths/%2Fblog%2Fpost/post/responses/201/content/application%2Fjson/schema/%24ref: #/components/schemas/BlogPost does not resolve to a valid object (Openapi3Parser::Error::InvalidData)
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node_factory/field.rb:86:in `data'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node_factory/field.rb:63:in `data'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node_factory/field.rb:43:in `node'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node_factory/reference.rb:48:in `build_node'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node_factory/object.rb:51:in `node'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node/placeholder.rb:49:in `node'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node/placeholder.rb:12:in `resolve'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node/object.rb:35:in `[]'
	from /Users/developer/.rbenv/versions/2.6.9/lib/ruby/gems/2.6.0/gems/openapi3_parser-0.9.2/lib/openapi3_parser/node/media_type.rb:11:in `schema'
	from bug.rb:29:in `<main>'