title: jstree get_json returns false
body: <p>I'm using jsTree plugin to display tree.
  The tree is loaded using ajax to a div with ul-li structure. And here is how I initialize it.</p>
  
  <pre><code>$("#jseq_tree").jstree({
        "core" : { "check_callback" : true},
        "plugins" : ["dnd","wholerow"]
  }).jstree("open_all");
  </code></pre>
  
  <p>I'm trying to get all nodes from the tree on a button click like that:</p>
  
  <pre><code>$(document).on("click", "#sub_tree", function() {
      var v =$('#jseq_tree').jstree('get_json', -1);
      var mytext = JSON.stringify(v);
      alert(mytext);
  });
  </code></pre>
  
  <p>But it always alerts false. How can I make it work? Or how can I get all elements of the tree with their ids and children?</p>