Cannot new a custom field after installing the computed_custom_field plug-in.

[yingsheng658]

Description:
After installing Redmine plugin as https://github.com/annikoff/redmine_plugin_computed_custom_field,
Will get internal error when we would like to new a custom field.

Steps to reproduce the issue:
1.Download redmine_plugin_computed_custom_field.
2.Install plugin.
3.Migrate plugin.
4.Restart server.
5.Access the Redmine system.
6.New a custom field after installing the plug-in.

Describe the results you received:
Internal error when new a custom field after installing the plug-in.

Describe the results you expected:
Can new a custom field after installing the plug-in.

Version
1.OS:Windows server 2008.
2.Redmine version is v3.3.0-1.

Production log:

Started GET "/redmine/custom_fields/new?utf8=%E2%9C%93&type=IssueCustomField" for 127.0.0.1 at 2017-06-16 01:45:24 -0700
Processing by CustomFieldsController#new as HTML
Parameters: {"utf8"=>"?", "type"=>"IssueCustomField"}
Current user: user (id=1)
Rendered custom_fields/formats/_regexp.html.erb (0.0ms)
Rendered custom_fields/formats/_string.html.erb (0.0ms)
Rendered plugins/computed_custom_field/app/views/hooks/_view_custom_fields_form_upper_box.html.erb (15.6ms)
Rendered custom_fields/_form.html.erb (15.6ms)
Rendered custom_fields/new.html.erb within layouts/admin (15.6ms)
Completed 500 Internal Server Error in 62ms (ActiveRecord: 46.8ms)

ActionView::Template::Error (undefined method is_computed' for #IssueCustomField:0xb23ba68):
1: <% if custom_field.new_record? || custom_field.is_computed? %>
2:

<%= form.check_box :is_computed,
3: data: { enables: '#custom_field_formula, #available_cfs' },
4: disabled: !custom_field.new_record? %>

5:

<%= form.text_area :formula, rows: 7 %>

lib/redmine/views/labelled_form_builder.rb:34:incheck_box'
plugins/computed_custom_field/app/views/hooks/_view_custom_fields_form_upper_box.html.erb:2:in _plugins_computed_custom_field_app_views_hooks__view_custom_fields_form_upper_box_html_erb___558248688_63599544'
lib/redmine/hook/view_listener.rb:59:inblock (2 levels) in render_on'
lib/redmine/hook/view_listener.rb:57:in map'
lib/redmine/hook/view_listener.rb:57:inblock in render_on'
lib/redmine/hook.rb:61:in block (2 levels) in call_hook'
lib/redmine/hook.rb:61:ineach'
lib/redmine/hook.rb:61:in block in call_hook'
lib/redmine/hook.rb:58:intap'
lib/redmine/hook.rb:58:in call_hook'
lib/redmine/hook.rb:96:incall_hook'
app/views/custom_fields/_form.html.erb:20:in _app_views_custom_fields__form_html_erb__482887807_64258752'
app/views/custom_fields/new.html.erb:4:inblock in appviews_custom_fields_new_html_erb__350912484_64331700'
app/helpers/application_helper.rb:1051:in labelled_form_for'
app/views/custom_fields/new.html.erb:3:inappviews_custom_fields_new_html_erb__350912484_64331700'
lib/redmine/sudo_mode.rb:63:in sudo_mode'