amplify-cli: Failed to start API Mock endpoint Typerror

Describe the bug Trying to mock the API returns an error that is untraceable and not very unhelpful. Failed to start API Mock endpoint TypeError: Cannot read property '0' of null

Amplify CLI Version cli version: 4.21.0 amplify version: 3.0.11

To Reproduce mkdir amplify-bug
cd amplify-bug
amplify init
amplify add auth <= default config
amplify add api <= graphql default config

Use the following schema:

type Project
  @model
  @auth(
    rules: [
      { allow: groups, groups: ["Reg", "Admin"] }
      { allow: groups, groupsField: "client", operations: [read] }
    ]
  )
  @key(name: "ByClient", fields: ["client"], queryField: "projectsByClient")
  @key(name: "BySlug", fields: ["slug"], queryField: "projectsBySlug") {
  id: ID!
  title: String!
  slug: String!
  client: ClientGroupEnum!
  description: String
  logo: S3Object
  concepts: [Concept]
    @connection(keyName: "byConcept", fields: ["id"], sortField: "createdAt")
}

enum ClientGroupEnum {
  Reg
  Aux
  Tyt
}

type Concept
  @model(queries: null)
  @key(name: "byConcept", fields: ["projectID"])
  @auth(
    rules: [
      {
        allow: groups
        groups: ["Admin", "Reg"]
        operations: [create, update, delete]
      }
    ]
  ) {
  id: ID!
  name: String!
  projectID: ID!
  moodboard: S3Object
  pages: [Page]
    @connection(keyName: "byPage", fields: ["id"], sortField: "createdAt")
}

type Page
  @model(queries: null)
  @key(name: "byPage", fields: ["conceptID"])
  @auth(
    rules: [
      {
        allow: groups
        groups: ["Admin", "Reg"]
        operations: [create, update, delete]
      }
    ]
  ) {
  id: ID!
  name: String!
  size: PageSizeEnum!
  conceptID: ID!
  image: S3Object!
}

type S3Object {
  key: String!
  identityId: String!
}

enum PageSizeEnum {
  Small
  Medium
  Large
}

Expected behavior Mock API endpoint working

Desktop (please complete the following information):

• OS: Mac
• Node Version: v12.16.2