MD Settings: fix a couple errors

chrome/browser/resources/settings/settings_main/settings_main.js

 // Copyright 2015 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
  * 'settings-main' displays the selected settings page.
  */
 Polymer({
   is: 'settings-main',
@@ skipping 133 matching lines @@
     if (!this.currentRoute || this.currentRoute.subpage.length != 0 ||
         this.showPages_.about) {
       return 0;
     }
 
     var query = 'settings-section[section="' + this.currentRoute.section + '"]';
     var topSection = this.$$('settings-basic-page').$$(query);
     if (!topSection && this.showPages_.advanced)
       topSection = this.$$('settings-advanced-page').$$(query);
 
-    if (!topSection)
+    if (!topSection || !topSection.offsetParent)
       return 0;
 
     // Offset to the selected section (relative to the scrolling window).
     let sectionTop = topSection.offsetParent.offsetTop + topSection.offsetTop;
     // The height of the selected section and remaining content (sections).
     let heightOfShownSections = this.$.overscroll.offsetTop - sectionTop;
     return Math.max(0, this.parentNode.scrollHeight - heightOfShownSections);
   },
 
   /** @private */
@@ skipping 43 matching lines @@
 
   /**
    * @param {(boolean|undefined)} visibility
    * @return {boolean} True unless visibility is false.
    * @private
    */
   showAdvancedSettings_: function(visibility) {
     return visibility !== false;
   },
 });