diff -u b/core/misc/ajax.js b/core/misc/ajax.js
--- b/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -887,7 +887,7 @@
     // attachBehaviors() called on the new content from processing the response
     // commands is not sufficient, because behaviors from the entire form need
     // to be reattached.
-    if (this.$form) {
+    if (this.$form && $.contains(document, this.$form.get(0))) {
       var settings = this.settings || drupalSettings;
       Drupal.attachBehaviors(this.$form.get(0), settings);
     }