Comparison with nil

Processing by AssetPageController#show as */*
Completed 500 Internal Server Error in 49ms (ActiveRecord: 2.7ms)
  
ActionView::Template::Error (comparison of NilClass with 1270 failed):
    4:   - if @asset.ext
    5:     = "(" + @asset.ext + ")"
    6: 
    7:   - previews = @asset.thumbnails.preview_images
    8:   - if previews.any?
    9:     %br
   10:       = :preview.t + ':'
  
app/models/asset.rb:122:in `sort_by'
app/models/asset.rb:122:in `preview_images'
extensions/pages/asset_page/app/views/asset_page/_information.html.haml:7:in `_extensions_pages_asset_page_app_views_asset_page__information_html_haml___4053301782972737526_70168982178700'
extensions/pages/asset_page/app/views/asset_page/show.html.haml:8:in `_extensions_pages_asset_page_app_views_asset_page_show_html_haml___3304851899128385177_70168982955960'
app/controllers/common/application/before_filters.rb:5:in `render'
app/controllers/dispatch_controller.rb:14:in `dispatch'