message: prevent error for empty list
diff: @@ -759,8 +759,10 @@ module.exports = createReactClass({
  _getMessagesHeight() {
  const itemlist = this.refs.itemlist;
  const lastNode = itemlist.lastElementChild;
  + const lastNodeBottom = lastNode ? lastNode.offsetTop + lastNode.clientHeight : 0;
  + const firstNodeTop = itemlist.firstElementChild ? itemlist.firstElementChild.offsetTop : 0;
  // 18 is itemlist padding
  - return (lastNode.offsetTop + lastNode.clientHeight) - itemlist.firstElementChild.offsetTop + (18 * 2);
  + return lastNodeBottom - firstNodeTop + (18 * 2);
  },
  _topFromBottom(node) {
  