Fix a crash by setting a flag to true.

docshell/base/nsDocShell.cpp

@@ -938,6 +938,9 @@ nsDocShell::~nsDocShell()
 {
   MOZ_ASSERT(!mProfileTimelineRecording);
 
+  // Avoid notifying observers while we're in the dtor.
+  mIsBeingDestroyed = true;
+
   Destroy();
 
   nsCOMPtr<nsISHistoryInternal> shPrivate(do_QueryInterface(mSessionHistory));