The error occurs in this case

1. install new redmine and there is no project.
2. From top menu, click Project
3. From this project index page, click View All Issue
4. From that issue index page, click on New Issue Then the error 500 occurs with the following logs

Started GET "/redmine/issues/new" for 123.20.56.163 at 2016-07-23 07:41:55 +0000
Processing by IssuesController#new as HTML
Current user: admin (id=1)
Rendered issues/_attributes.html.erb (7.5ms)
Rendered issues/_form.html.erb (26.4ms)
Rendered issues/new.html.erb within layouts/base (28.5ms)
Completed 500 Internal Server Error in 100ms (ActiveRecord: 34.2ms)

ActionView::Template::Error (undefined method assignable_users' for nil:NilClass):
15: <% end %>
16:
17: <% if @issue.safe_attribute? 'assigned_to_id' %>
18: <p><%= f.select :assigned_to_id, principals_options_for_select(@issue.assignable_users, @issue.assigned_to), :include_blank => true, :required => @issue.required_attribute?('assigned_to_id') %></p>
19: <% end %>
20:
21: <% if @issue.safe_attribute?('category_id') && @issue.project.issue_categories.any? %>
app/models/issue.rb:857:inassignable_users'
app/views/issues/attributes.html.erb:18:in `block in _app_views_issuesattributes_html_erb3587757851355343547_69925371132800'
app/helpers/application_helper.rb:1058:in labelled_fields_for'
app/views/issues/_attributes.html.erb:1:in_app_views_issuesattributes_html_erb_3587757851355343547_69925371132800'
app/views/issues/_form.html.erb:42:in `block in _app_views_issuesform_html_erb_3675110379538404660_69925371413960'
app/helpers/application_helper.rb:1058:in labelled_fields_for'
app/views/issues/_form.html.erb:1:in_app_views_issuesform_html_erb_3675110379538404660_69925371413960'
app/views/issues/new.html.erb:11:in `block in _app_views_issues_new_html_erb153899361600342295_69925371381020'
app/helpers/application_helper.rb:1051:in labelled_form_for'
app/views/issues/new.html.erb:5:inapp_views_issues_new_html_erb_153899361600342295_69925371381020'
app/controllers/issues_controller.rb:129:in block (2 levels) in new'
app/controllers/issues_controller.rb:128:innew'
lib/redmine/sudo_mode.rb:63:in `sudo_mode'