undefined method `+' for nil:NilClass

[boianmihailov]

Rendering wiki/edit_permissions

ActionView::TemplateError (undefined method `+' for nil:NilClass) on line #90 of vendor/plugins/redmine_wiki_permissions/app/views/wiki/edit_permissions.html.erb:
87: }
88:
89: .wiki_permission_roles {
90: background: url(<%= ActionController::Base.relative_url_root + "/images/database_key.png" %>) no-repeat;
91: padding-left: 18px;
92: }
93: </style>