title: Cannot read property 'concat' of null in select2
body: <p>I am trying to get all data including parents &amp; childs tp show with select2. I found an <a href="http://jsfiddle.net/56be9zop/135/" rel="nofollow noreferrer">example</a> which is useful for it. When I try to change "children" to "childPools", I got an error </p>
  
  <blockquote>
    <p>Uncaught TypeError: Cannot read property 'concat' of null</p>
  </blockquote>
  
  <pre><code> var FRUIT_GROUPS = [
              {
                  id: '',
                  text: 'Citrus',
                  childPools: [
                      { id: 'c1', text: 'Grapefruit' },
                      { id: 'c2', text: 'Orange' },
                      { id: 'c3', text: 'Lemon' },
                      { id: 'c4', text: 'Lime' }
                  ]
              },
              {
                  id: '',
                  text: 'Other',
                  childPools: [
                      { id: 'o1', text: 'Apple' },
                      { id: 'o2', text: 'Mango' },
                      { id: 'o3', text: 'Banana' }
                  ]
              }
          ];
  
          $('#fruitSelect').select2({
              multiple: false,
              placeholder: "Select fruits...",
              data: FRUIT_GROUPS,
              query: function (options) {
                  var selectedIds = options.element.select2('val');
                  var data = jQuery.extend(true, {}, FRUIT_GROUPS);
                  var selectableGroups = $.map(data, function (group) {
                      var areAllChildrenSelected = true,
                          parentMatchTerm = false,
                          anyChildMatchTerm = false;
                      if (group.text.toLowerCase().indexOf(options.term.toLowerCase()) &gt;= 0) {
                          parentMatchTerm = true;
                      }
                      var i = group.childPools.length
                      while (i--) {
                          var child = group.childPools[i];
  
                          if (selectedIds.indexOf(child.id) &lt; 0) {
                              areAllChildrenSelected = false;
                          };
  
                          if (options.term == '' || (child.text.toLowerCase().indexOf(options.term.toLowerCase()) &gt;= 0)) {
                              anyChildMatchTerm = true;
                          }
                          else if (!parentMatchTerm) {
                              var index = group.childPools.indexOf(child);
                              if (index &gt; -1) {
                                  group.childPools.splice(index, 1);
                              };
                          };
                      };
  
                      return (!areAllChildrenSelected &amp;&amp; (parentMatchTerm || anyChildMatchTerm)) ? group : null;
                  });
  
                  options.callback({ results: selectableGroups });
              }
          }).on('select2-selecting', function (e) {
              var $select = $(this);
              if (e.val == '') {
                  e.preventDefault();
                  $select.select2('data', $select.select2('data').concat(e.choice.childPools));
                  $select.select2('close');
              }
          });
  </code></pre>
  
  <p>I got the error on the following line where I'm concatenating the selection to the <code>select2('data')</code>.</p>
  
  <pre><code>$select.select2('data',$select.select2('data').concat(e.choice.children));
  </code></pre>
  
  <p>Any idea about this error?</p>