title: Channel Invokemethod from native android not invoking the flutter method
body: <p>Basically I'm invoking a payment sdk in native android(kotlin) using platform channel. SDK is initialised successfully. After payment transaction I will receive the payment status in a kotlin file which actually extends sdk class files (not an activity). From this callback method I need to pass the status to the flutter dart code. Everything runs successfully without any error but while passing the payment status flutter method is not invoked.     </p>
  
  <p>Flutter dart code.</p>
  
  <pre><code>@override
  void initState() {
    // TODO: implement initState
    super.initState();
  
    platform.setMethodCallHandler(_handlePaymentResponse); // To call from native android after payment response.
  
  }
  
  // Invoked from Native Android after payment response.
  Future&lt;dynamic&gt; _handlePaymentResponse(MethodCall call) async {
    print('_handlePaymentResponse method called');
    switch(call.method) {
      case "message":
        debugPrint('From Native====' + call.arguments);
        return new Future.value("");
    }
  }
  
  
  // On button click this method will be invoked.
  Future&lt;void&gt; _initializeSDK() async {
  
    print('Token $token');
  
    try {
      await platform.invokeMethod('callPaymentSDK');
    } on PlatformException catch (e) {
      print("Failed to initialize SDK : '${e.message}'.");
    }
  
  }
  Native Android : Paymentstatus.kt 
  
  class PaymentResponse() : LibraryPaymentStatusProtocol, Parcelable {
  
      //val channel = MethodChannel(flutterView, MainActivity.CHANNEL)
  
      var backgroundFlutterView: FlutterNativeView? = null
  
      override fun paymentStatus(p0: String?, p1: Activity?) {
  
          if (p1 != null) {
              Handler(Looper.getMainLooper()).post {
                  Log.v("PaymentResponse", "Main thread called")
                  backgroundFlutterView = FlutterNativeView(p1, true)
                  val channel = MethodChannel(backgroundFlutterView, MainActivity.CHANNEL)
                  channel.invokeMethod("message", p0); // Invoking Flutter method 
              }
  
          }
  
  
      }
  }
  </code></pre>