title: `no implicit conversion of nil into String` wih ruby2d
body: <p>So for practice with ruby, I am creating a library on top of ruby2d, but one problem.<br>
  I keep getting an error, I think setting up a window, here is the full error:</p>
  <pre><code>C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/ruby2d-0.11.3/lib/ruby2d/window.rb:476:in `exist?': no implicit conversion of nil into String (TypeError)
          from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/ruby2d-0.11.3/lib/ruby2d/window.rb:476:in `add_controller_mappings'
          from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/ruby2d-0.11.3/lib/ruby2d/window.rb:629:in `ext_show'
          from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/ruby2d-0.11.3/lib/ruby2d/window.rb:629:in `show'
          from C:/Users/Admin/Desktop/RubyProjects/RSGE/rsge_window.rb:19:in `mainloop'
          from c:/Users/Admin/Desktop/RubyProjects/test.rb:4:in `&lt;main&gt;'
  
  </code></pre>
  <p>it shows that something is wrong in ruby2d (a graphics library for ruby)</p>
  <p>here is the code for my test file that uses my library</p>
  <pre><code>require './RSGE/rsge'
  
  root = Window.new(800, 800)
  root.mainloop
  </code></pre>
  <p>and here is my window class:</p>
  <pre><code>require 'ruby2d'
  require_relative 'rsge_colors'
  
  class Window
      def initialize (w, h)
          @width = w.to_i
          @height = h.to_i
          @title = 'Blank Window'
          @background = $WHITE
      end
  
      def config
          set width: @width, height: @height 
          set background: @background
          set title: @title
      end
  
      def mainloop
          show
      end 
  end
  </code></pre>
  <p>if your wondering what rsge_colors is, here it is</p>
  <pre><code>$RED    = 'red'
  $GREEN  = 'green'
  $BLUE   = 'blue'
  $ORANGE = 'orange'
  $PURPLE = 'purple'
  $YELLOW = 'yellow'
  $WHITE  = 'white'
  $BLACK  = 'black'
  
  </code></pre>
  <p>Why am I getting this error, and how can I fix it?
  Thank you for any help you can give me!</p>