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

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

Index: chrome/browser/resources/settings/settings_page/main_page_behavior.js
diff --git a/chrome/browser/resources/settings/settings_page/main_page_behavior.js b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
index ce1581225272332145660dd1e31012a22fad65e4..fd15c75f0ae7886d02fbac1ad530f248703ce354 100644
--- a/chrome/browser/resources/settings/settings_page/main_page_behavior.js
+++ b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
@@ -66,6 +66,9 @@ var MainPageBehaviorImpl = {
    * @private
    */
   inSearchModeChanged_: function(inSearchMode) {
+    if (!this.isAttached)
+      return;
+
     if (!inSearchMode)
       this.tryTransitionToSection_(!settings.lastRouteChangeWasPopstate());
   },