Ruby instance variable always nil outside `def initialize`

I have a controller method that passes an instance variable @show_column to a grid class. The problem is that although the value of the instance variable is available in def initialize of the grid class, it is nil outside def initialize:

The controller method:

  def index
    @show_column = (current_user && current_user.admin?)
    @grid = UsersGrid.new(params[:users_grid], @show_column) do |scope|
      scope.where(admin: false).page(params[:page]).per_page(30)
    end
    @grid.assets
  end

The grid class:

class UsersGrid
  include Datagrid
  attr_reader :show_column

  def initialize(*params, show_column)
    super *params
    @show_column = show_column
  end

  scope do
    User.order("users.created_at desc")
  end

  column(:abc, :header => "abc?", :html => true, :if => proc {@show_column == true}) do |user|
    image_tag("abc.png", title: "abc") if user.abc
  end
end

The problem is at the end with: proc {@show_column == true}. This doesn't work because @show_column is always nil. I've used a debugger to try to find some additional information. I shows that @show_column is set correctly in the controller; its value is true as it should be given the user I logged in with. Also, inside def initialize its value is true. However, outside def initialize in the grid class, its value is nil. Any ideas how to solve this?