NoMethodError (You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+):
  app/models/parkinglot.rb:51:in `inc_total_values'
  app/models/parkinglot.rb:30:in `create_callback'
  app/controllers/admin/parkinglots_controller.rb:10:in `block in create'
  app/controllers/admin/parkinglots_controller.rb:9:in `each'
  app/controllers/admin/parkinglots_controller.rb:9:in `create'