ActionView::Template::Error (no implicit conversion of nil into String):

[rkd0308]

When I changed the settings of Knowledgebase Plugin on webpage,
I can't access to the Knowledgebase page.
it recalls internal error.

How can I fix this problem?
Actually I don't know ruby on rails..
But I can follow up your advice.
Thanks,

This is Production_log

ActionView::Template::Error (no implicit conversion of nil into String):
25:
26: <% end %>
27:
28: <%= format_article_summary(article, summary_format, :truncate => truncate_length ) %>
29:
30:
31: <% end %>
plugins/redmine_knowledgebase/app/helpers/knowledgebase_helper.rb:38:in +' plugins/redmine_knowledgebase/app/helpers/knowledgebase_helper.rb:38:in format_article_summary'
plugins/redmine_knowledgebase/app/views/articles/_list.html.erb:28:in block in _plugins_redmine_knowledgebase_app_views_articles__list_html_erb__1382619362784318947_69848792222480' plugins/redmine_knowledgebase/app/views/articles/_list.html.erb:1:in each'
plugins/redmine_knowledgebase/app/views/articles/_list.html.erb:1:in _plugins_redmine_knowledgebase_app_views_articles__list_html_erb__1382619362784318947_69848792222480' plugins/redmine_knowledgebase/app/views/articles/index.html.erb:37:in _plugins_redmine_knowledgebase_app_views_articles_index_html_erb__3470066968146296770_69848803865440'
lib/redmine/sudo_mode.rb:63:in sudo_mode' plugins/redmine_dmsf/lib/redmine_dmsf/webdav/custom_middleware.rb:68:in call'

and here is my version

Environment:
Redmine version 3.4.10.stable
Ruby version 2.2.0-p0 (2014-12-25) [x86_64-linux]
Rails version 4.2.11.1
Environment production
Database adapter Mysql2
SCM:
Subversion 1.6.11
Git 1.7.1
Filesystem
Redmine plugins:
clipboard_image_paste 1.10
computed_custom_field 1.0.7
issuecloser 0.0.1
projects_tree_view 0.0.8
redmine_contacts 4.2.4
redmine_contacts_helpdesk 4.0.2
redmine_dmsf 1.6.2
redmine_email_images 0.1.1
redmine_image_clipboard_paste 3.3.0
redmine_knowledgebase 3.2.1
redmine_lightbox2 0.4.4
redmine_login_attempts_limit 0.1.0
redmine_login_audit 0.3.3
redmine_mail_reminder 3.0.0.0001
redmine_monitoring_controlling 0.1.1