reattempt #239, closes #243

This is how it should have been done in the first place. An additional
condition proves enough to guard against unscheduled events, that do not
even have time slots defined.

app/views/schedule/_unscheduled_events.html.haml

 - @unscheduled_events.each do |event|
-  %li.unscheduled-event{ :id => "event_#{event.id}",
-          :"data-update-url" => schedule_update_event_path(:id => event.id),
-          :"data-height" => (event.time_slots * 20 - 7) }
-    = link_to event.title, "#"
+  - if event.time_slots != nil
+    %li.unscheduled-event{ :id => "event_#{event.id}",
+            :"data-update-url" => schedule_update_event_path(:id => event.id),
+            :"data-height" => (event.time_slots * 20 - 7) }
+      = link_to event.title, "#"