Redmine

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:in `assignable_users'
app/views/issues/_attributes.html.erb:18:in `block in app_views_issues_attributes_html_erb___3587757851355343547_69925371132800'
app/helpers/application_helper.rb:1058:in `labelled_fields_for'
app/views/issues/_attributes.html.erb:1:in `_app_views_issues__attributes_html_erb___3587757851355343547_69925371132800'
app/views/issues/_form.html.erb:42:in `block in app_views_issues_form_html_erb___3675110379538404660_69925371413960'
app/helpers/application_helper.rb:1058:in `labelled_fields_for'
app/views/issues/_form.html.erb:1:in `_app_views_issues__form_html_erb___3675110379538404660_69925371413960'
app/views/issues/new.html.erb:11:in `block in app_views_issues_new_html_erb_153899361600342295_69925371381020'
app/helpers/application_helper.rb:1051:in `labelled_form_for'
app/views/issues/new.html.erb:5:in `_app_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:in `new'
lib/redmine/sudo_mode.rb:63:in `sudo_mode'