Fix get_css_classes on new default tops without localtops

/close #195

templates/protocol-tops-include.html

@@ -4,6 +4,7 @@
         {% for default_top in protocol.protocoltype.default_tops %}
             {% if not default_top.is_at_end() %}
                 {% set localtop = default_top.get_localtop(protocol) %}
+                {% if localtop is not none %}
                   <li class="{{localtop.get_css_classes()|join(" ")}}" {% if localtop.is_expandable() %} id="localtop-{{localtop.id}}" title="{{localtop.description}}"{% endif %}>
                       {{default_top.name}}
                       {% if not protocol.is_done() and has_modify_right %}
@@ -17,6 +18,7 @@
                       {% endif %}
                   </li>
                 {% endif %}
+            {% endif %}
         {% endfor %}
     {% endif %}
     {% for top in protocol.tops %}