500 error encountered when browsing files

Created by: tildaslashams

I just upgraded my Gitlab instance from 3.1 to 4-0-stable , when I browse any code file I get 500 error and here is the log :

Completed 500 Internal Server Error in 120ms

ActionView::Template::Error (Failed to get header.):
    9:   .file_content.code
    10:     - unless blob.empty?
    11:       %div{class: user_color_scheme_class}
    12:         = raw blob.colorize(formatter: :gitlab)
    13:     - else
    14:       %p.nothing_here_message Empty file
  app/models/tree.rb:5:in `colorize'
  app/views/tree/blob/_text.html.haml:12:in `_app_views_tree_blob__text_html_haml__857522388875419420_55507340'
  app/views/tree/_blob.html.haml:9:in `_app_views_tree__blob_html_haml___2533693189977314418_55458360'
  app/views/tree/_tree.html.haml:15:in `_app_views_tree__tree_html_haml___1664782134279665212_55472320'
  app/views/tree/show.js.haml:3:in `_app_views_tree_show_js_haml__4569430623983790506_55381760'

}