undefined method `invokable_worker_methods'

[idemarinis]

While updating access point configuration I receive the following error message in backgroundrb log

<NoMethodError: undefined method `invokable_worker_methods' for nil:NilClass>

/var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/server/lib/master_worker.rb:129:in worker_methods' /var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/server/lib/master_worker.rb:107:inasync_method_invoke' /var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/server/lib/master_worker.rb:38:in receive_data' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_parser.rb:44:inextract' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_parser.rb:26:in loop' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_parser.rb:26:inextract' /var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/server/lib/master_worker.rb:32:in receive_data' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:230:inread_external_socket' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:222:in handle_external_messages' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:196:inhandle_read_event' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:192:in each' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:192:inhandle_read_event' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:146:in start_reactor' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:139:inloop' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_core.rb:139:in start_reactor' /var/rails/OpenWISP-Manager/vendor/bundle/ruby/1.8/bundler/gems/packet-64d67edaae1a/lib/packet/packet_master.rb:21:inrun' /var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/server/lib/master_proxy.rb:14:in initialize' /var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/lib/backgroundrb/bdrb_start_stop.rb:109:innew' /var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/lib/backgroundrb/bdrb_start_stop.rb:109:in start_bdrb' /var/rails/OpenWISP-Manager/vendor/plugins/backgroundrb/lib/backgroundrb/bdrb_start_stop.rb:16:instart' /var/rails/OpenWISP-Manager/script/backgroundrb:24