Get Android activity result in Flutter

I have a thirdparty Android control that can't be used in Flutter directly. I put it in an Android activity. Then, using information from https://flutter.io/docs/development/platform-integration/platform-channels#step-3b-add-an-android-platform-specific-implementation-using-kotlin, I can successfully launch that activity and perform some actions there. The only part that doesn't work is sending results back from the activity.

Flutter code:

void showDialog() async
    {
        try {
            final Map<String, List<double>> result = await platform.invokeMethod('show_dialog',
                <String, String>{
                    'address': widget.user.address
                });
            widget.user.address = result.keys.toList()[0];
        } on PlatformException catch (e) {
            print('Failed to pick address: ${e.message}.');
        }
    }

Android code:

class MainActivity: FlutterActivity() {
  private val CHANNEL = "dialog"
  private lateinit var _result: MethodChannel.Result
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    GeneratedPluginRegistrant.registerWith(this)
    MethodChannel(flutterView, CHANNEL).setMethodCallHandler { call, result ->
      if (call.method == "show_dialog") {
          _result = result
          val intent = Intent(this@MainActivity, DialogActivity::class.java)
          intent.putExtra("address", call.argument<String>("address"))
          startActivityForResult(intent, 9689)
      }
      else result.notImplemented()
    }
  }
  override fun onActivityResult(requestCode: Int, result: Int, intent: Intent?) {
      if(requestCode != 9689)
          return super.onActivityResult(requestCode, result, intent)
      if (result == Activity.RESULT_OK) {
          _result.success(mapOf(intent!!.getStringExtra("address") to
              listOf(intent.getDoubleExtra("latitude", 0.0),
                  intent.getDoubleExtra("longitude", 0.0))))
      }
      else
          _result.success(null)
  }
}

What's the problem? Breakpoint on the line widget.user.address = result.keys.toList()[0]; is never reached, suggesting the result is never sent back.