issue #260 fixed

sails/src/assets/scripts/modules/student/views/session.html

@@ -6,7 +6,7 @@
         <button class="btn btn-default" btn-radio="'new'" ng-model="section">
         <button class="btn btn-default" btn-radio="'new'" ng-model="section">
           <span class="glyphicon glyphicon-record"></span> {{ 'new_session' | translate }}
           <span class="glyphicon glyphicon-record"></span> {{ 'new_session' | translate }}
         </button>
         </button>
-        <button class="btn btn-default" btn-radio="'previous'" ng-model="section" ng-click="load_tries()">
+        <button class="btn btn-default" btn-radio="'previous'" ng-model="section" ng-click="load_tries()" ng-if="wsessions.length > 0">
           <span class="glyphicon glyphicon-transfer"></span> {{ 'previous_sessions' | translate }}
           <span class="glyphicon glyphicon-transfer"></span> {{ 'previous_sessions' | translate }}
         </button>
         </button>
       </div>
       </div>