Index: source/gui/IGUIObject.h
===================================================================
--- source/gui/IGUIObject.h
+++ source/gui/IGUIObject.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2017 Wildfire Games.
+/* Copyright (C) 2018 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -506,8 +506,8 @@
 	// Internal storage for registered script handlers.
 	std::map<CStr, JS::Heap<JSObject*> >	m_ScriptHandlers;
 
-	// Cached JSObject representing this GUI object
-	JS::PersistentRootedObject				m_JSObject;
+	// Cached JSValue representing this GUI object
+	JS::Heap<JS::Value> m_JSObjectValue;
 };
 
 
Index: source/gui/IGUIObject.cpp
===================================================================
--- source/gui/IGUIObject.cpp
+++ source/gui/IGUIObject.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2016 Wildfire Games.
+/* Copyright (C) 2018 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -502,14 +502,15 @@
 	JSContext* cx = m_pGUI->GetScriptInterface()->GetContext();
 	JSAutoRequest rq(cx);
 	// Cache the object when somebody first asks for it, because otherwise
-	// we end up doing far too much object allocation. TODO: Would be nice to
-	// not have these objects hang around forever using up memory, though.
-	if (!m_JSObject.initialized())
+	// we end up doing far too much object allocation.
+
+	if (m_JSObjectValue.isUndefined())
 	{
-		m_JSObject.init(cx, m_pGUI->GetScriptInterface()->CreateCustomObject("GUIObject"));
-		JS_SetPrivate(m_JSObject.get(), this);
+		m_JSObjectValue = JS::Heap<JS::Value>(JS::ObjectValue(*m_pGUI->GetScriptInterface()->CreateCustomObject("GUIObject")));
+		JS_SetPrivate(&m_JSObjectValue.toObject(), this);
 	}
-	return m_JSObject.get();
+
+	return &(m_JSObjectValue.toObject());
 }
 
 CStr IGUIObject::GetPresentableName() const
@@ -542,6 +543,8 @@
 
 void IGUIObject::TraceMember(JSTracer* trc)
 {
+	JS_CallValueTracer(trc, &m_JSObjectValue, "IGUIObject::m_JSObjectValue");
+
 	for (std::pair<const CStr, JS::Heap<JSObject*>>& handler : m_ScriptHandlers)
 		JS_CallObjectTracer(trc, &handler.second, "IGUIObject::m_ScriptHandlers");
 }