Fixed possibility to set ical feed to none.

app/assets/javascripts/app/views/generic/ical_feed.jst.eco

@@ -9,7 +9,7 @@
   </label>
   <div class="u-inlineBlock u-positionOrigin">
     <select class="form-control form-control--small js-list">
-      <option>-</option>
+      <option value="">-</option>
       <% for url, name of @ical_feeds: %>
         <option value="<%= url %>" <% if @attribute.value is url: %>selected<%end%>><%- @T(name) %></option>
       <% end %>

app/models/calendar.rb

@@ -209,7 +209,10 @@ returns
   def sync(without_save = nil)
     return if !ical_url
     begin
-      events = Calendar.parse(ical_url)
+      events = {}
+      if ical_url && !ical_url.empty?
+        events = Calendar.parse(ical_url)
+      end
 
       # sync with public_holidays
       if !public_holidays