ServiceWorker::from can handle null ServiceWorkers

Source/modules/serviceworkers/ServiceWorker.cpp

Index: Source/modules/serviceworkers/ServiceWorker.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorker.cpp b/Source/modules/serviceworkers/ServiceWorker.cpp
index 43c7506c95214bb9082f545862becb68f13e4aa3..b1de87b41bdaa829ed13beb2cea9383ad9f28c18 100644
--- a/Source/modules/serviceworkers/ServiceWorker.cpp
+++ b/Source/modules/serviceworkers/ServiceWorker.cpp
@@ -140,6 +140,9 @@ const AtomicString& ServiceWorker::state() const
 
 PassRefPtr<ServiceWorker> ServiceWorker::from(ExecutionContext* executionContext, WebType* worker)
 {
+    if (!worker)
+        return PassRefPtr<ServiceWorker>();
+
     blink::WebServiceWorkerProxy* proxy = worker->proxy();
     ServiceWorker* existingServiceWorker = proxy ? proxy->unwrap() : 0;
     if (existingServiceWorker) {