Handle a non-existing promise when receiving a signin error in the SupervisedUserListData.

chrome/browser/resources/options/supervised_user_list_data.js

Index: chrome/browser/resources/options/supervised_user_list_data.js
diff --git a/chrome/browser/resources/options/supervised_user_list_data.js b/chrome/browser/resources/options/supervised_user_list_data.js
index 407634778bf64f6236be10f19fc01684f79079b9..f73feca63f0421b663ee1f78cf5363acea3aece2 100644
--- a/chrome/browser/resources/options/supervised_user_list_data.js
+++ b/chrome/browser/resources/options/supervised_user_list_data.js
@@ -31,8 +31,8 @@ cr.define('options', function() {
    *       }
    * @private
    */
-  SupervisedUserListData.prototype.receiveExistingSupervisedUsers_ = function(
-    supervisedUsers) {
+  SupervisedUserListData.prototype.receiveExistingSupervisedUsers_ =
+      function(supervisedUsers) {
     if (!this.promise_) {
       this.onDataChanged_(supervisedUsers);
       return;
@@ -46,7 +46,9 @@ cr.define('options', function() {
    * @private
    */
   SupervisedUserListData.prototype.onSigninError_ = function() {
-    assert(this.promise_);
+    if (!this.promise_) {
+      return;
+    }
     this.reject_();
     this.resetPromise_();
   };