Prevent Javascript error if the page will be unlocked

Make an undefined check to the locked status label which is only available in page tree view. If user is closing the tab in page edit view (or any other view) it will result in a javascript error.

(cherry picked from commit bc1a1c1)

app/views/alchemy/admin/pages/unlock.js.erb

@@ -15,6 +15,6 @@
     }
     locked_page_icon.outerHTML = locked_page_icon_content
   }
-  document.querySelector("#page_<%= @page.id -%> .page_status.locked").remove()
+  document.querySelector("#page_<%= @page.id -%> .page_status.locked")?.remove()
   Alchemy.growl('<%= flash[:notice] -%>')
 })()