Failed to open event stream on Flutter's EventChannel ( Parameter specified as non-null is null)

Question

Here's my MainActivity.kt:

override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
    super.configureFlutterEngine(flutterEngine)

    EventChannel(flutterEngine.dartExecutor.binaryMessenger, EVENT_CHANNEL).setStreamHandler(
            object : EventChannel.StreamHandler {
                override fun onListen(args: Any, events: EventChannel.EventSink) {
                    Log.d(TAG, "adding listener for $EVENT_CHANNEL")
                    events.success(true)
                }

                override fun onCancel(args: Any) {
                    Log.d(TAG, "cancelling listener for $EVENT_CHANNEL")
                }
            }
    )

Then on Flutter I open it like this:

  final stream = const EventChannel('com.my_app.event_channel');

Then on initState:

widget.stream.receiveBroadcastStream().listen((event) {
  print("@@@@@@@@@@@@@@@@@@@@@@@@@@ received event channel!");
});

What I get:

E/EventChannel#com.my_app.event_channel(16346): Failed to open event stream
E/EventChannel#com.my_app.event_channel(16346): java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter args
E/EventChannel#com.my_app.event_channel(16346):     at com.my_app.flutter_app.MainActivity$configureFlutterEngine$1.onListen(Unknown Source:2)
E/EventChannel#com.my_app.event_channel(16346):     at io.flutter.plugin.common.EventChannel$IncomingStreamRequestHandler.onListen(EventChannel.java:188)
E/EventChannel#com.my_app.event_channel(16346):     at io.flutter.plugin.common.EventChannel$IncomingStreamRequestHandler.onMessage(EventChannel.java:167)
E/EventChannel#com.my_app.event_channel(16346):     at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:85)
E/EventChannel#com.my_app.event_channel(16346):     at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:692)
E/EventChannel#com.my_app.event_channel(16346):     at android.os.MessageQueue.nativePollOnce(Native Method)
E/EventChannel#com.my_app.event_channel(16346):     at android.os.MessageQueue.next(MessageQueue.java:325)
E/EventChannel#com.my_app.event_channel(16346):     at android.os.Looper.loop(Looper.java:142)
E/EventChannel#com.my_app.event_channel(16346):     at android.app.ActivityThread.main(ActivityThread.java:6617)
E/EventChannel#com.my_app.event_channel(16346):     at java.lang.reflect.Method.invoke(Native Method)
E/EventChannel#com.my_app.event_channel(16346):     at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
E/EventChannel#com.my_app.event_channel(16346):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:769)

Looks like onListen is being called with null data. But I do not pass any data. What's happening?

Solution

Just add ? on args: Any so it looks like this: args: Any?, example:

override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
    super.configureFlutterEngine(flutterEngine)

    EventChannel(flutterEngine.dartExecutor.binaryMessenger, EVENT_CHANNEL).setStreamHandler(
            object : EventChannel.StreamHandler {
                override fun onListen(args: Any?, events: EventChannel.EventSink) {
                    Log.d(TAG, "adding listener for $EVENT_CHANNEL")
                    events.success(true)
                }

                override fun onCancel(args: Any?) {
                    Log.d(TAG, "cancelling listener for $EVENT_CHANNEL")
                }
            }
    )

This way it will accept null args. The way it was before was forcing args to be non null, but when flutter opens a channel without any name like

widget.stream.receiveBroadcastStream()

it sends a null arg to that code. Another solution would be to send something:

widget.stream.receiveBroadcastStream("something")

but it's better to make the possibility for Any to be null if you might expect no parameters