From c843f8e7f963670bcf939ff689bcaab6119ea92c Mon Sep 17 00:00:00 2001
From: blagoev <lubo@blagoev.com>
Date: Sat, 9 Sep 2017 02:00:46 +0300
Subject: [PATCH] Validate callback was registered on unregistration

---
 src/js_sync.hpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/js_sync.hpp b/src/js_sync.hpp
index 168ba535..80fc1fcb 100644
--- a/src/js_sync.hpp
+++ b/src/js_sync.hpp
@@ -375,6 +375,10 @@ void SessionClass<T>::remove_progress_notification(ContextType ctx, FunctionType
     validate_argument_count(argc, 1);
     auto callback_function = Value::validated_to_function(ctx, arguments[0], "callback");
     auto syncSessionProp = Object::get_property(ctx, callback_function, "syncSession");
+    if (Value::is_undefined(ctx, syncSessionProp) || Value::is_null(ctx, syncSessionProp)) {
+        return;
+    }
+
     auto syncSession = Value::validated_to_object(ctx, syncSessionProp);
     auto registrationToken = Object::get_property(ctx, callback_function, "registrationToken");