devise、rails_admin、cancancanを使って特定の管理者のみuserを新規登録できるようなアプリにしたい

前提・実現したいこと

Ruby on railsで特定のuserのみがログインできるようなアプリを作ろうとしています。
現在、devise,rails_admin,cancancanのgemを導入して、
userテーブルにはadmin_flg :blooeanカラムを作成し、
admin_flgがtrueのユーザーだけが/adminへアクセスできるようにするところまでできました。
しかし、管理画面から新しいuserを登録しようとするとNoMethodErrorになります。

発生している問題・エラーメッセージ

error
1NoMethodError in RailsAdmin::Main#new
2Showing/Users/*****/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/bundler/gems/rails_admin-692ae9910be9/app/views/rails_admin/main/index.html.haml where line #25 raised:
3
4undefined method `properties' for nil:NilClass
5
6undefined method `type_for_attribute' for Store:Class

terminal
1Processing by RailsAdmin::MainController#index as HTML
2  Parameters: {"model_name"=>"user"}
3  User Load (0.4ms)  SELECT `users`.* FROM `users` WHERE `users`.`id` = 1 ORDER BY `users`.`id` ASC LIMIT 1
4  ↳ config/initializers/rails_admin.rb:7:in `block (2 levels) in <top (required)>'
5  Rendering /Users/********/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/bundler/gems/rails_admin-692ae9910be9/app/views/rails_admin/main/index.html.haml within layouts/rails_admin/application
6  Rendered /Users/********/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/bundler/gems/rails_admin-692ae9910be9/app/views/rails_admin/main/index.html.haml within layouts/rails_admin/application (Duration: 4.5ms | Allocations: 3663)
7Completed 500 Internal Server Error in 10ms (ActiveRecord: 0.4ms | Allocations: 6428)
8
9
10  
11ActionView::Template::Error (undefined method `properties' for nil:NilClass):
12    22: - content_for :contextual_tabs do
13    23:   - if checkboxes
14    24:     = bulk_menu
15    25:   - if filterable_fields.present?
16    26:     %li.dropdown{style: 'float:right'}
17    27:       %a.dropdown-toggle{href: '#', :'data-toggle' => "dropdown"}
18    28:         = t('admin.misc.add_filter')
19  
20rails_admin (692ae9910be9) lib/rails_admin/config/fields/types/belongs_to_association.rb:19:in `block in <class:BelongsToAssociation>'

該当のソースコード

rails_admin.rb
1RailsAdmin.config do |config|
2
3  ### Popular gems integration
4
5  # == Devise ==
6  config.authenticate_with do
7    warden.authenticate! scope: :user
8  end
9  config.current_user_method(&:current_user)
10
11  # == CancanCan ==
12  config.authorize_with :cancancan
13
14  ## == Pundit ==
15  # config.authorize_with :pundit
16
17  ## == PaperTrail ==
18  # config.audit_with :paper_trail, 'User', 'PaperTrail::Version' # PaperTrail >= 3.0.0
19
20  ### More at https://github.com/sferik/rails_admin/wiki/Base-configuration
21
22  ## == Gravatar integration ==
23  ## To disable Gravatar integration in Navigation Bar set to false
24  # config.show_gravatar = true
25
26  config.actions do
27    dashboard                     # mandatory
28    index                         # mandatory
29    new
30    export
31    bulk_delete
32    show
33    edit
34    delete
35    show_in_app
36
37    ## With an audit adapter, you can add:
38    # history_index
39    # history_show
40  end
41end
42