title: ng-file-upload files null when clicking upload button for the second time
body: <p>I'm using ng-file-upload for my angular project, here's the html code:</p>
  
  <pre><code>&lt;div class="form-group"&gt;
      &lt;button class="btn btn-warning" ngf-select="imagesSelected($files)" multiple="multiple" ngf-pattern="'image/*'" accept="image/jpeg,image/png"&gt;选择图片&lt;/button&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>And this is my javascript code:</p>
  
  <pre><code>$scope.imagesSelected = function (files) {
      if (files.length &gt; 0) {
          angular.forEach(files, function (imageFile, index) {
              $scope.upload = Upload.upload({
                  url: '/upload_image',
                  method: 'POST',
                  file: imageFile,
                  data: {
                      'fileName': imageFile.name
                  }
              }).success(function (response, status, headers, config) {
                  ...
              });
          });
      }
  };
  </code></pre>
  
  <p>The first time when click the button to select image files, image files get uploaded immediately after selected, which is what I expected. But the second time when I click the upload button, the console has this error pointing to the if (files.length > 0) line:</p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'length' of null
  </code></pre>
  
  <p>and the file selecting window never show up, 
  the third time I click the upload button it works fine again, fourth time not, and so on...
  The version of ng-file-upload is 9.0.4, is this bug of the lib has not been fixed or did I make some mistake? Thanks.</p>