NoMethodError: undefined method `gender' for nil:NilClass
  app/views/volunteer/update.slim:18:in `block (2 levels) in singleton class'
    = f.select(:gender, :options => [['男性', 'male'], ['女性', 'female']], class: 'u-full-width', value: @user.gender)
  app/views/volunteer/update.slim:13:in `block in singleton class'
    = form_for :volunteer, '/volunteer/update', :id => 'update', :class => 'ui form' do |f|
  app/controllers/volunteer.rb:86:in `block (2 levels) in <top (required)>'
    render 'volunteer/update', layout: 'site'
  lib/identity_map_middleware.rb:9:in `block in call'
    @app.call(env)
  lib/identity_map_middleware.rb:8:in `call'
    ::DataMapper.repository(@name) do
...
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NoMethodError: undefined method `gender' for nil:NilClass