diff --git a/app/assets/javascripts/editor/editor.js.erb b/app/assets/javascripts/editor/editor.js.erb
index d50f663a..52b9c7fb 100644
--- a/app/assets/javascripts/editor/editor.js.erb
+++ b/app/assets/javascripts/editor/editor.js.erb
@@ -1011,7 +1011,10 @@ var CodeOceanEditor = {
     },
 
     applyChanges: function (delta, active_file) {
-        const editor = this.editor_for_file.get(active_file.id)
+        const editor = this.editor_for_file.get(active_file.id);
+        if (editor === undefined) {
+            return;
+        }
         editor.session.doc.applyDeltas([delta]);
     },