From 4f7e431137caffc0c1cc8deee361893a7eabe70c Mon Sep 17 00:00:00 2001
From: wolfbeast <mcwerewolf@gmail.com>
Date: Tue, 11 Dec 2018 23:17:50 +0100
Subject: Clear weak pointers in VTT shutdown observers.

---
 dom/html/TextTrackManager.cpp | 9 ++++++++-
 dom/html/TextTrackManager.h   | 8 ++++++--
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/dom/html/TextTrackManager.cpp b/dom/html/TextTrackManager.cpp
index 4266575f7..cc14858b6 100644
--- a/dom/html/TextTrackManager.cpp
+++ b/dom/html/TextTrackManager.cpp
@@ -29,6 +29,13 @@ namespace dom {
 
 NS_IMPL_ISUPPORTS(TextTrackManager::ShutdownObserverProxy, nsIObserver);
 
+void
+TextTrackManager::ShutdownObserverProxy::Unregister()
+{
+  nsContentUtils::UnregisterShutdownObserver(this);
+  mManager = nullptr;
+}
+
 CompareTextTracks::CompareTextTracks(HTMLMediaElement* aMediaElement)
 {
   mMediaElement = aMediaElement;
@@ -137,7 +144,7 @@ TextTrackManager::TextTrackManager(HTMLMediaElement *aMediaElement)
 TextTrackManager::~TextTrackManager()
 {
   WEBVTT_LOG("%p ~TextTrackManager",this);
-  nsContentUtils::UnregisterShutdownObserver(mShutdownProxy);
+  mShutdownProxy->Unregister();
 }
 
 TextTrackList*
diff --git a/dom/html/TextTrackManager.h b/dom/html/TextTrackManager.h
index d20707346..4ad1a57a7 100644
--- a/dom/html/TextTrackManager.h
+++ b/dom/html/TextTrackManager.h
@@ -170,11 +170,15 @@ private:
     {
       MOZ_ASSERT(NS_IsMainThread());
       if (strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0) {
-        nsContentUtils::UnregisterShutdownObserver(this);
-        mManager->NotifyShutdown();
+        if (mManager) {
+          mManager->NotifyShutdown();
+        }
+        Unregister();
       }
       return NS_OK;
     }
+    
+    void Unregister();
 
   private:
     ~ShutdownObserverProxy() {};
-- 
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