Fix compiler error for MessageHandler

[fengyuxx]

No description provided.

[szotp]

Hi @fengyuxx thanks for your MR. Since this repo was created we discovered another problem which is not yet fixed by this repo. You may need to:

1. Copy autogenerated registerWithRegistry into AppDelegate.m

2. Comment out FLTFirebaseMessagingPlugin.

  //[FLTFirebaseMessagingPlugin registerWithRegistrar:[registry registrarForPlugin:@"FLTFirebaseMessagingPlugin"]];
  [FlutterApnsPlugin registerWithRegistrar:[registry registrarForPlugin:@"FlutterApnsPlugin"]];

3. Sync registerWithRegistry every time you add new plugins.

It's tricky because the order of registration matters: when FlutterApnsPlugin registered before FLTFirebaseMessagingPlugin then everything worked fine. We probably need to use method swizzling to nullify FLTFirebaseMessagingPlugin registerWithRegistrar or fork the firebase plugin to disable it on iOS.