error 500 on file listing in one repository

Created by: bastianh

The content of the errorlog:

Started GET "/xxx/tree/master" for 127.0.0.1 at 2012-11-19 10:46:20 +0100
Processing by TreeController#show as HTML
  Parameters: {"project_id"=>"xxx", "id"=>"master"}
  Rendered shared/_ref_switcher.html.haml (0.7ms)
  Rendered tree/_head.html.haml (1.2ms)
  Rendered tree/_tree_item.html.haml (0.5ms)
  Rendered tree/_tree_item.html.haml (0.4ms)
  Rendered tree/_tree_item.html.haml (0.4ms)
  Rendered tree/_submodule_item.html.haml (0.6ms)
  Rendered tree/_tree.html.haml (3.1ms)
  Rendered tree/show.html.haml within layouts/project_resource (4.6ms)
Completed 500 Internal Server Error in 22ms

ActionView::Template::Error (can't convert nil into String):
    30:           %td
    31:           %td
    32: 
    33:       = render_tree(tree.contents)
    34: 
    35:     - if tree.readme
    36:       = render "tree/readme", readme: tree.readme
  app/helpers/tree_helper.rb:18:in `+'
  app/helpers/tree_helper.rb:18:in `block in render_tree'
  app/helpers/tree_helper.rb:15:in `each'
  app/helpers/tree_helper.rb:15:in `render_tree'
  app/views/tree/_tree.html.haml:33:in `_app_views_tree__tree_html_haml___2644747827922859863_98765360'
  app/views/tree/show.html.haml:3:in `_app_views_tree_show_html_haml__3651807114895933032_99133840'
  app/controllers/tree_controller.rb:17:in `show'