ChannelEventHandler class Null safety

Represents a class to notify various channel event.

This is base class that you can mixin, so that you can register on SendbirdSdk.addChannelEventHandler. Typically you want to use this class as following:

class MyHandler with ChannelEventHandler {
  @override
  void onMessageReceived(BaseChannel channel, BaseMessage message) {
     //user logic here when new message is arrived
  }
  ...
}