Schema processing fails when "allOf" references to nullable object

Describe the bug
When using a schema that contains an "allOf" reference to a nullable object, the generation fails with the message "Cannot take allOf a non-object".

OpenAPI Spec File
Document to recreate:

openapi: 3.0.1
info:
  title: Title
  description: Description
  version: 1.0.0
servers:
  - url: /api
paths:
  /item:
    get:
      operationId: item
      responses:
        "200":
          description: Successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Item'
components:
  schemas:
    RelatedItemSuper:
      type: object
      nullable: true
      properties:
        propertyA:
          type: string
    RelatedItem:
      type: object
      allOf:
        - $ref: '#/components/schemas/RelatedItemSuper'
        - type: object
          properties:
            propertyB:
              type: string
    Item:
      type: object
      nullable: false
      properties:
        key:
          type: string
        relatedItem:
          $ref: '#/components/schemas/RelatedItem'

A real world example can be found here.

Desktop (please complete the following information):

• OS: Linux
• Python Version: 3.11.2
• openapi-python-client version: 0.21.1

Additional context
Changing "nullable" to "false" on "RelatedItem" in the above example works but does not seem to be a suitable solution as the construct used is legitimate, or is it not?

Start by running openapi-python-client 0.21.1 against the supplied OpenAPI 3.0.1 document and trace the schema-processing path that emits "Cannot take allOf a non-object." Use the nullable RelatedItemSuper and RelatedItem definitions as the regression case; done means client generation succeeds without requiring nullable to be changed to false.