MD Settings: Fix CrOS /help/details edge case for MainPageBehavior.

chrome/browser/resources/settings/settings_page/main_page_behavior.js

 // Copyright 2016 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.
 
 /**
  * Responds to route changes by expanding, collapsing, or scrolling to sections
  * on the page. Expanded sections take up the full height of the container. At
  * most one section should be expanded at any given time.
  * @polymerBehavior MainPageBehavior
  */
@@ skipping 48 matching lines @@
     else
       this.tryTransitionToSection_(scrollToSection);
   },
 
   /**
    * When exiting search mode, we need to make another attempt to scroll to
    * the correct section, since it has just been re-rendered.
    * @private
    */
   inSearchModeChanged_: function(inSearchMode) {
+    if (!this.isAttached)
+      return;
+
     if (!inSearchMode)
       this.tryTransitionToSection_(!settings.lastRouteChangeWasPopstate());
   },
 
   /**
    * If possible, transitions to the current route's section (by expanding or
    * scrolling to it). If another transition is running, finishes or cancels
    * that one, then schedules this function again. This ensures the current
    * section is quickly shown, without getting the page into a broken state --
    * if currentRoute changes in between calls, just transition to the new route.
@@ skipping 216 matching lines @@
     return /** @type {?SettingsSectionElement} */(
         this.$$('settings-section[section="' + section + '"]'));
   },
 };
 
 /** @polymerBehavior */
 var MainPageBehavior = [
   settings.RouteObserverBehavior,
   MainPageBehaviorImpl,
 ];