getEventHandler method
~english Get the presence event handler.
Param identifier The custom handler identifier.
Return The presence event handler. ~end
~chinese 获取 Presence 事件句柄。
Param identifier 事件对应 ID.
Return Presence 事件。 ~end
Implementation
ChatPresenceEventHandler? getEventHandler(String identifier) {
  return _eventHandlesMap[identifier];
}