Fix some ChromeVox Closure compiler errors.

chrome/browser/resources/chromeos/chromevox/walkers/abstract_node_walker.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview A base class for walkers that have a concept of lowest-level
  * node. Base classes must override the stopNodeDescent method to define
  * what a lowest-level node is. Then this walker will use those nodes as the
  * set of valid CursorSelections.
  */
@@ skipping 21 matching lines @@
   this.wasBegin_ = false;
 };
 goog.inherits(cvox.AbstractNodeWalker, cvox.AbstractWalker);
 
 /**
  * @override
  */
 cvox.AbstractNodeWalker.prototype.next = function(sel) {
   var r = sel.isReversed();
   var node = sel.end.node || document.body;
-
+  if (!node) {
+    return null;
+  }
   do {
     node = cvox.DomUtil.directedNextLeafLikeNode(node, r,
         goog.bind(this.stopNodeDescent, this));
     if (!node) {
       return null;
     }
     // and repeat all of the above until we have a node that is not empty
   } while (node && !cvox.DomUtil.hasContent(node));
 
   return cvox.CursorSelection.fromNode(node).setReversed(r);
@@ skipping 27 matching lines @@
     this.wasBegin_ = true;
   }
 
   var node = sel.start.node;
 
   while (node != document.body && node.parentNode &&
       this.stopNodeDescent(node.parentNode)) {
     node = node.parentNode;
   }
 
-  while (!this.stopNodeDescent(node)) {
+  while (node && !this.stopNodeDescent(node)) {
     node = cvox.DomUtil.directedFirstChild(node, r);
   }
 
   var n = cvox.CursorSelection.fromNode(node);
   if (!cvox.DomUtil.hasContent(node)) {
     n = this.next(/** @type {!cvox.CursorSelection} */
         (cvox.CursorSelection.fromNode(node)).setReversed(r));
   }
   if (n) {
     return n.setReversed(r);
   }
   return this.begin({reversed: r});
 };
 
 /**
  * Returns true if this is "a leaf node" or lower. That is,
  * it is at the lowest valid level or lower for this granularity.
  * RESTRICTION: true for a node => true for all child nodes
  * RESTRICTION: true if node has no children
  * @param {!Node} node The node to check.
  * @return {boolean} true if this is at the "leaf node" level or lower
  * for this granularity.
  * @protected
  */
 cvox.AbstractNodeWalker.prototype.stopNodeDescent = goog.abstractMethod;