adding track by to dynamic selects.

Author: JorgeVilladaNovaip

src/builder/mw-form-question-builder.html

@@ -24,7 +24,7 @@
                 <div class="form-group mw-question-type" ng-repeat="opt in ctrl.options.additionalSelects">
                     <mw-label label-for="question-{{opt.key}}-{{ctrl.id}}" label-class="col-sm-2 control-label"  label-key="{{opt.label}}"></mw-label>
                     <div class="col-sm-10 form-inline">
-                        <select ng-attr-id="question-{{opt.key}}-{{ctrl.id}}" ng-options="sel.label for sel in opt.selects" ng-model="ctrl.question[opt.key]"  class="form-control" ng-disabled="ctrl.readOnly" required></select>
+                        <select ng-attr-id="question-{{opt.key}}-{{ctrl.id}}" ng-options="sel.label for sel in opt.selects track by sel.key" ng-model="ctrl.question[opt.key]"  class="form-control" ng-disabled="ctrl.readOnly" required></select>
 
                     </div>
                 </div>