DOMWindow::navigator should return navigator w/o frame when detached.

Source/modules/serviceworkers/NavigatorServiceWorker.cpp

Index: Source/modules/serviceworkers/NavigatorServiceWorker.cpp
diff --git a/Source/modules/serviceworkers/NavigatorServiceWorker.cpp b/Source/modules/serviceworkers/NavigatorServiceWorker.cpp
index d2552311f9a3ba290dd69584d2e2b505c29e230f..cd79bf694cfddd862ae565bcd69329700e6f6906 100644
--- a/Source/modules/serviceworkers/NavigatorServiceWorker.cpp
+++ b/Source/modules/serviceworkers/NavigatorServiceWorker.cpp
@@ -26,8 +26,10 @@ NavigatorServiceWorker* NavigatorServiceWorker::from(Document& document)
 {
     if (!document.frame() || !document.frame()->domWindow())
         return nullptr;
-    Navigator& navigator = *document.frame()->domWindow()->navigator();
-    return &from(navigator);
+    Navigator* navigator = document.frame()->domWindow()->navigator();
+    if (!navigator)
+        return nullptr;
+    return &from(*navigator);
 }
 
 NavigatorServiceWorker& NavigatorServiceWorker::from(Navigator& navigator)