diff --git a/ext/fg/js/popup-proxy-host.js b/ext/fg/js/popup-proxy-host.js
index 157097de..cb385cc2 100644
--- a/ext/fg/js/popup-proxy-host.js
+++ b/ext/fg/js/popup-proxy-host.js
@@ -114,8 +114,7 @@ class PopupProxyHost {
         ++this._nextId;
         const popup = new Popup(id, depth, this._frameIdPromise);
         if (parent !== null) {
-            popup.parent = parent;
-            parent.child = popup;
+            popup.setParent(parent);
         }
         this._popups.set(id, popup);
         return {popup, id};
diff --git a/ext/fg/js/popup.js b/ext/fg/js/popup.js
index de2c7863..fafc15aa 100644
--- a/ext/fg/js/popup.js
+++ b/ext/fg/js/popup.js
@@ -113,6 +113,20 @@ class Popup {
 
     // Popup-only public functions
 
+    setParent(parent) {
+        if (parent === null) {
+            throw new Error('Cannot set popup parent to null');
+        }
+        if (this.parent !== null) {
+            throw new Error('Popup already has a parent');
+        }
+        if (parent.child !== null) {
+            throw new Error('Cannot parent popup to another popup which already has a child');
+        }
+        this.parent = parent;
+        parent.child = this;
+    }
+
     isVisible() {
         return this.isInjected && (this.visibleOverride !== null ? this.visibleOverride : this.visible);
     }