Bug 999341 - Null check the observer service in nsDocument::OnPageHide; r=smaug

content/base/src/nsDocument.cpp

@@ -8877,12 +8877,14 @@ nsDocument::OnPageHide(bool aPersisted,
 
   // Dispatch observer notification to notify observers page is hidden.
   nsCOMPtr<nsIObserverService> os = mozilla::services::GetObserverService();
-  nsIPrincipal *principal = GetPrincipal();
-  os->NotifyObservers(static_cast<nsIDocument*>(this),
-                      nsContentUtils::IsSystemPrincipal(principal) ?
-                        "chrome-page-hidden" :
-                        "content-page-hidden",
-                      nullptr);
+  if (os) {
+    nsIPrincipal* principal = GetPrincipal();
+    os->NotifyObservers(static_cast<nsIDocument*>(this),
+                        nsContentUtils::IsSystemPrincipal(principal) ?
+                          "chrome-page-hidden" :
+                          "content-page-hidden",
+                        nullptr);
+  }
 
   DispatchPageTransition(target, NS_LITERAL_STRING("pagehide"), aPersisted);