NoMethodError in RailsAdmin::Main#nestable

[dkobia]

I'm trying to drag/sort items with a nestable_tree organized with Ancestry and encountering the error below:

Environment

Rails: 5.1.6
Ruby: 2.4.1

config:

rails_admin do
  nestable_tree({
    position_field: :position,
    max_depth: 3
  })
end

Model has t.integer :position and t.string :ancestry, with ancestry gem installed.

Error

NoMethodError in RailsAdmin::Main#nestable

Showing /.rvm/gems/ruby-2.4.1/gems/rails_admin_nestable-0.3.2/app/views/rails_admin/main/nestable.html.haml where line #11 raised:
undefined method `map' for nil:NilClass

Framework Trace
rails_admin_nestable (0.3.2) lib/rails_admin_nestable/helper.rb:4:in `nested_tree_nodes'
rails_admin_nestable (0.3.2) app/views/rails_admin/main/nestable.html.haml:11:in `___sers__rvm_gems_ruby_______gems_rails_admin_nestable_______app_views_rails_admin_main_nestable_html_haml___1225494274636441108_70308752925720'
actionview (5.1.6) lib/action_view/template.rb:157:in `block in render'
activesupport (5.1.6) lib/active_support/notifications.rb:168:in `instrument'
actionview (5.1.6) lib/action_view/template.rb:352:in `instrument_render_template'
actionview (5.1.6) lib/action_view/template.rb:155:in `render'
actionview (5.1.6) lib/action_view/renderer/template_renderer.rb:52:in `block (2 levels) in render_template'
actionview (5.1.6) lib/action_view/renderer/abstract_renderer.rb:42:in `block in instrument'
activesupport (5.1.6) lib/active_support/notifications.rb:166:in `block in instrument'
activesupport (5.1.6) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (5.1.6) lib/active_support/notifications.rb:166:in `instrument'
actionview (5.1.6) lib/action_view/renderer/abstract_renderer.rb:41:in `instrument'
actionview (5.1.6) lib/action_view/renderer/template_renderer.rb:51:in `block in render_template'
actionview (5.1.6) lib/action_view/renderer/template_renderer.rb:59:in `render_with_layout'
actionview (5.1.6) lib/action_view/renderer/template_renderer.rb:50:in `render_template'
actionview (5.1.6) lib/action_view/renderer/template_renderer.rb:14:in `render'
actionview (5.1.6) lib/action_view/renderer/renderer.rb:42:in `render_template'
actionview (5.1.6) lib/action_view/renderer/renderer.rb:23:in `render'
actionview (5.1.6) lib/action_view/rendering.rb:103:in `_render_template'
actionpack (5.1.6) lib/action_controller/metal/streaming.rb:217:in `_render_template'
actionview (5.1.6) lib/action_view/rendering.rb:83:in `render_to_body'
actionpack (5.1.6) lib/action_controller/metal/rendering.rb:52:in `render_to_body'
actionpack (5.1.6) lib/action_controller/metal/renderers.rb:141:in `render_to_body'
actionpack (5.1.6) lib/abstract_controller/rendering.rb:24:in `render'
actionpack (5.1.6) lib/action_controller/metal/rendering.rb:36:in `render'
actionpack (5.1.6) lib/action_controller/metal/instrumentation.rb:44:in `block (2 levels) in render'
activesupport (5.1.6) lib/active_support/core_ext/benchmark.rb:12:in `block in ms'
/.rvm/rubies/ruby-2.4.1/lib/ruby/2.4.0/benchmark.rb:308:in `realtime'
activesupport (5.1.6) lib/active_support/core_ext/benchmark.rb:12:in `ms'
actionpack (5.1.6) lib/action_controller/metal/instrumentation.rb:44:in `block in render'
actionpack (5.1.6) lib/action_controller/metal/instrumentation.rb:87:in `cleanup_view_runtime'
activerecord (5.1.6) lib/active_record/railties/controller_runtime.rb:29:in `cleanup_view_runtime'
actionpack (5.1.6) lib/action_controller/metal/instrumentation.rb:43:in `render'
remotipart (1.3.1) lib/remotipart/render_overrides.rb:16:in `render_with_remotipart'
rails_admin_nestable (0.3.2) lib/rails_admin_nestable/nestable.rb:66:in `block (2 levels) in <class:Nestable>'
rails_admin (1.3.0) app/controllers/rails_admin/main_controller.rb:22:in `instance_eval'
rails_admin (1.3.0) app/controllers/rails_admin/main_controller.rb:22:in `nestable'
actionpack (5.1.6) lib/action_controller/metal/basic_implicit_render.rb:4:in `send_action'
actionpack (5.1.6) lib/abstract_controller/base.rb:186:in `process_action'
actionpack (5.1.6) lib/action_controller/metal/rendering.rb:30:in `process_action'
actionpack (5.1.6) lib/abstract_controller/callbacks.rb:20:in `block in process_action'
activesupport (5.1.6) lib/active_support/callbacks.rb:131:in `run_callbacks'
actionpack (5.1.6) lib/abstract_controller/callbacks.rb:19:in `process_action'
actionpack (5.1.6) lib/action_controller/metal/rescue.rb:20:in `process_action'
actionpack (5.1.6) lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
activesupport (5.1.6) lib/active_support/notifications.rb:166:in `block in instrument'
activesupport (5.1.6) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (5.1.6) lib/active_support/notifications.rb:166:in `instrument'
actionpack (5.1.6) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
actionpack (5.1.6) lib/action_controller/metal/params_wrapper.rb:252:in `process_action'
activerecord (5.1.6) lib/active_record/railties/controller_runtime.rb:22:in `process_action'
actionpack (5.1.6) lib/abstract_controller/base.rb:124:in `process'
actionview (5.1.6) lib/action_view/rendering.rb:30:in `process'
actionpack (5.1.6) lib/action_controller/metal.rb:189:in `dispatch'
actionpack (5.1.6) lib/action_controller/metal.rb:253:in `dispatch'
actionpack (5.1.6) lib/action_dispatch/routing/route_set.rb:49:in `dispatch'
actionpack (5.1.6) lib/action_dispatch/routing/route_set.rb:31:in `serve'
actionpack (5.1.6) lib/action_dispatch/journey/router.rb:50:in `block in serve'
actionpack (5.1.6) lib/action_dispatch/journey/router.rb:33:in `each'
actionpack (5.1.6) lib/action_dispatch/journey/router.rb:33:in `serve'
actionpack (5.1.6) lib/action_dispatch/routing/route_set.rb:844:in `call'
railties (5.1.6) lib/rails/engine.rb:522:in `call'
railties (5.1.6) lib/rails/railtie.rb:185:in `public_send'
railties (5.1.6) lib/rails/railtie.rb:185:in `method_missing'
actionpack (5.1.6) lib/action_dispatch/routing/mapper.rb:17:in `block in <class:Constraints>'
actionpack (5.1.6) lib/action_dispatch/routing/mapper.rb:46:in `serve'
actionpack (5.1.6) lib/action_dispatch/journey/router.rb:50:in `block in serve'
actionpack (5.1.6) lib/action_dispatch/journey/router.rb:33:in `each'
actionpack (5.1.6) lib/action_dispatch/journey/router.rb:33:in `serve'
actionpack (5.1.6) lib/action_dispatch/routing/route_set.rb:844:in `call'
rack-pjax (1.0.0) lib/rack/pjax.rb:12:in `call'
remotipart (1.3.1) lib/remotipart/middleware.rb:32:in `call'
warden (1.2.7) lib/warden/manager.rb:36:in `block in call'
warden (1.2.7) lib/warden/manager.rb:35:in `catch'
warden (1.2.7) lib/warden/manager.rb:35:in `call'
rack (2.0.4) lib/rack/etag.rb:25:in `call'
rack (2.0.4) lib/rack/conditional_get.rb:38:in `call'
rack (2.0.4) lib/rack/head.rb:12:in `call'
rack (2.0.4) lib/rack/session/abstract/id.rb:232:in `context'
rack (2.0.4) lib/rack/session/abstract/id.rb:226:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/cookies.rb:613:in `call'
activerecord (5.1.6) lib/active_record/migration.rb:556:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/callbacks.rb:26:in `block in call'
activesupport (5.1.6) lib/active_support/callbacks.rb:97:in `run_callbacks'
actionpack (5.1.6) lib/action_dispatch/middleware/callbacks.rb:24:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/debug_exceptions.rb:59:in `call'
web-console (3.5.1) lib/web_console/middleware.rb:135:in `call_app'
web-console (3.5.1) lib/web_console/middleware.rb:28:in `block in call'
web-console (3.5.1) lib/web_console/middleware.rb:18:in `catch'
web-console (3.5.1) lib/web_console/middleware.rb:18:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/show_exceptions.rb:31:in `call'
railties (5.1.6) lib/rails/rack/logger.rb:36:in `call_app'
railties (5.1.6) lib/rails/rack/logger.rb:24:in `block in call'
activesupport (5.1.6) lib/active_support/tagged_logging.rb:69:in `block in tagged'
activesupport (5.1.6) lib/active_support/tagged_logging.rb:26:in `tagged'
activesupport (5.1.6) lib/active_support/tagged_logging.rb:69:in `tagged'
railties (5.1.6) lib/rails/rack/logger.rb:24:in `call'
sprockets-rails (3.2.1) lib/sprockets/rails/quiet_assets.rb:13:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/remote_ip.rb:79:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/request_id.rb:25:in `call'
rack (2.0.4) lib/rack/method_override.rb:22:in `call'
rack (2.0.4) lib/rack/runtime.rb:22:in `call'
activesupport (5.1.6) lib/active_support/cache/strategy/local_cache_middleware.rb:27:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.1.6) lib/action_dispatch/middleware/static.rb:125:in `call'
rack (2.0.4) lib/rack/sendfile.rb:111:in `call'
railties (5.1.6) lib/rails/engine.rb:522:in `call'
puma (3.11.3) lib/puma/configuration.rb:225:in `call'
puma (3.11.3) lib/puma/server.rb:624:in `handle_request'
puma (3.11.3) lib/puma/server.rb:438:in `process_client'
puma (3.11.3) lib/puma/server.rb:302:in `block in run'
puma (3.11.3) lib/puma/thread_pool.rb:120:in `block in spawn_thread'

[DanTaiko]

I've got the similar error on Rails 6.

See the fix I suggested in the #69