Fix Error 500 when creating Wiki pages with hyphens or spaces

Closes gitlab-com/support-forum#874

CHANGELOG

@@ -10,6 +10,9 @@ v 8.11.0 (unreleased)
 v 8.10.1 (unreleased)
  - Fix bug where replies to commit notes displayed in the MR discussion tab wouldn't show up on the commit page
 
+v 8.10.1 (unreleased)
+  - Fix Error 500 when creating Wiki pages with hyphens or spaces
+
 v 8.10.0
   - Fix profile activity heatmap to show correct day name (eanplatter)
   - Speed up ExternalWikiHelper#get_project_wiki_path

app/views/layouts/project.html.haml

@@ -5,8 +5,8 @@
 
 - content_for :scripts_body_top do
   - project = @target_project || @project
-  - if @project_wiki && @page
-    - markdown_preview_path = namespace_project_wiki_markdown_preview_path(project.namespace, project, @page.title)
+  - if @project_wiki && @page && @page.slug.present?
+    - markdown_preview_path = namespace_project_wiki_markdown_preview_path(project.namespace, project, @page.slug)
   - else
     - markdown_preview_path = markdown_preview_namespace_project_path(project.namespace, project)
   - if current_user