message: Fix a crash when a DispatchQueueScheduler is destroyed without ever setting the callback
diff: @@ -136,8 +136,8 @@ public:
  bool can_deliver_notifications() const noexcept override { return true; }
  private:
  - dispatch_queue_t m_queue;
  - void (^m_callback)();
  + dispatch_queue_t m_queue = nullptr;
  + void (^m_callback)() = nullptr;
  };
  static const void* c_queue_key = &c_queue_key;
  