Redmine

I have done upgrade from 0.9.3 to 1.2.1. Everything works fine, but issue list. I got an error:
ActionView::TemplateError (undefined method `-' for nil:NilClass) on line #28 of app/views/issues/_list.rhtml:

I found similar topics here, but in my case db-migrate went great - zero problems.

Full error output:

Processing IssuesController#index (for 192.168.1.10 at 2011-08-16 05:09:55) [GET]
  Parameters: {"project_id"=>"cyfmpi", "action"=>"index", "controller"=>"issues"}
Rendering template within layouts/base
Rendering issues/index.rhtml

ActionView::TemplateError (undefined method `-' for nil:NilClass) on line #28 of app/views/issues/_list.rhtml:
25:             </tr>
26:             <% previous_group = group %>
27:   <% end %>
28:     <tr id="issue-<%= issue.id %>" class="hascontextmenu <%= cycle('odd', 'even') %> <%= issue.css_classes %> <%= level > 0 ? "idnt idnt-#{level}" : nil %>">
29:       <td class="checkbox hide-when-print"><%= check_box_tag("ids[]", issue.id, false, :id => nil) %></td>
30:             <td class="id"><%= link_to issue.id, :controller => 'issues', :action => 'show', :id => issue %></td>
31:         <% query.columns.each do |column| %><%= content_tag 'td', column_content(column, issue), :class => column.css_classes %><% end %>

    app/models/issue.rb:578:in `css_classes'
    app/views/issues/_list.rhtml:28:in `_run_rhtml_app47views47issues47_list46rhtml_locals_issues_list_object_query'
    app/helpers/issues_helper.rb:27:in `issue_list'
    app/helpers/issues_helper.rb:23:in `each'
    app/helpers/issues_helper.rb:23:in `issue_list'
    app/views/issues/_list.rhtml:16:in `_run_rhtml_app47views47issues47_list46rhtml_locals_issues_list_object_query'
    app/views/issues/_list.rhtml:1:in `_run_rhtml_app47views47issues47_list46rhtml_locals_issues_list_object_query'
    app/views/issues/index.rhtml:51:in `_run_rhtml_app47views47issues47index46rhtml'
    app/controllers/issues_controller.rb:92:in `index'
    app/controllers/issues_controller.rb:91:in `index'
    /opt/ruby1.8/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
    /opt/ruby1.8/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:162:in `start'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:95:in `start'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:92:in `each'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:92:in `start'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:23:in `start'
    /opt/ruby1.8/lib/ruby/1.8/webrick/server.rb:82:in `start'

Rendering /home/lighttpd/vaduz.lan/redmine/public/500.html (500 Internal Server Error)