Fix error on non-issue pages with comment areas.

app/views/projects/_md_preview.html.haml

@@ -8,7 +8,7 @@
         %a.js-md-preview-button{ href: "#md-preview-holder", tabindex: -1 }
           Preview
 
-      - if @issue.confidential?
+      - if defined?(@issue) && @issue.confidential?
         %li.confidential-issue-warning
           = icon('warning')
           %span This is a confidential issue. Your comment will not be visible to the public.