Dynamic actor isolation (implemented in Swift 6) inserts runtime assertions into Swift callbacks if the callback is supposed to be @MainActor-isolated. The Flutter message channels API (and the FlutterBinaryMessenger class) typically takes a taskQueue parameter which if not nil, the FlutterEngine is going to invoke the callback on the thread represented by the taskQueue (and that thread usually isn't the main thread).

So when the user does this in their Swift app:

func didInitializeImplicitFlutterEngine(_ engineBridge: any FlutterImplicitEngineBridge) {
  GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry)
  let channel = FlutterMethodChannel(
    name: "channel",
    binaryMessenger: engineBridge.applicationRegistrar.messenger(),
    codec: FlutterJSONMethodCodec.sharedInstance(),
    taskQueue: engineBridge.applicationRegistrar.messenger().makeBackgroundTaskQueue?(),
  )
  
  channel.setMethodCallHandler { (call: FlutterMethodCall, result: FlutterResult) -> Void in
    result(nil)
  }
}

and send a message to this channel in the dart program:

  const MethodChannel('channel', JSONMethodCodec()).invokeMethod('message');

The program crashes (even in production) with this stacktrace, when "dynamic actor isolation" is on:

Impact

I couldn't find any related github issues.

List of code on Github that uses the makeBackgroundTaskQueue API.

"Dynamic actor isolation" is on by default if you set the Swift language version to 6:

Workaround

Tell the compiler the closure is @Sendable

  channel.setMethodCallHandler { @Sendable (call: FlutterMethodCall, result: FlutterResult) -> Void in
    result(nil)
  }

Turn off dynamic actor isolation in release mode.