title: the factory value is changed to be null in controller randomly, however I injected it to controller
body: <p>I have an controller called <code>myController</code> as the following: </p>
  
  <pre><code>'use strict';
  
  app.controller('myController', ['$rootScope', '$scope', 'myService',
   function ($rootScope, $scope, myService) { 
       $scope.myService = myService;
  
       $rootScope.$on('kpi:submit', function (event, data) {
              $window.sessionStorage['prevCall'] = JSON.stringify(data.prevCall);
              $window.sessionStorage['currCall'] = JSON.stringify(data.currentCall);
              $window.sessionStorage['nextCall'] = JSON.stringify(data.nextCall);
  
              myService.prevCall = $scope.prevCall = data.prevCall;
              myService.currentCall = $scope.currentCall = data.currentCall;
              myService.nextCall = $scope.nextCall = data.nextCall;
       });
  }]);
  </code></pre>
  
  <p>And The <code>myService</code> factory, as the following: </p>
  
  <pre><code>'use strict';
  
  app.factory('myService', ['$http', '$q', function ($http, $q) {     
          this.currentCall;
          this.prevCall;
          this.nextCall;
  
          // some function
          return {
              myFunction : myFunction
          };
  }]);
  </code></pre>
  
  <p>The <code>kpi:submit</code> is called, when I click on <code>Submit</code> button. It's working well, but sometimes I got the following error: </p>
  
  <pre><code>TypeError: Cannot set property 'prevCall' of null
  </code></pre>
  
  <p>So, Why the <code>myService</code> factory is changed to <code>null</code> suddenly, any suggested solution, please ? </p>