fixed example

examples/network/15_dot_language_playground.html

@@ -80,7 +80,8 @@
 </table>
 
 <script type="text/javascript">
-  var network, data;
+  var network = null;
+  var data = null;
 
   var btnDraw = document.getElementById('draw');
   var txtData = document.getElementById('data');