Summary

Fix deadlock when calling removeEventHandler() from another thread while also calling it from within an event handler (reactor thread).

Problem

A deadlock occurred due to lock ordering inversion:

• Thread 1 (external): NotificationCenter::_mutex → NObserver::_mutex
• Thread 2 (handler): NObserver::_mutex → NotificationCenter::_mutex

When Thread 1 called removeObserver() holding _mutex and trying to call disable(), while Thread 2 was in notify() holding NObserver::_mutex and the handler called removeObserver(), both threads would deadlock.

Solution

1. Make _pObject atomic in NObserver and Observer to allow safe concurrent access without always requiring the mutex
2. Call disable() outside the lock in NotificationCenter::removeObserver() and destructor

The fix breaks the nested lock acquisition by releasing NotificationCenter::_mutex before calling disable().

Changes

• NObserver.h: Changed C* _pObject to std::atomic<C*> _pObject
• Observer.h: Same change (deprecated but still used)
• NotificationCenter.cpp: Modified removeObserver() and destructor to call disable() after releasing the lock

Fixes #4970