Accessing DecorView of the Flutter Activity in android native using a self created plugin

Question

I want to access the flutter activity's decorView in my native code. I created a plugin which give me access to the apis in the android native code. One of the api is

public void setActivity(Activity activity){
    Log.d("ActivityFromFlutter", activity.getLocalClassName());
    ...
    ...
}

I am using it like this in my flutter plugin:

private void setupPlugin(Context context, BinaryMessenger messenger, Registrar registrar) {
    if (registrar != null) {
        this.activity = ((Activity) registrar.activeContext());
        this.nativeSdk.setActivity(this.activity);
    }
}

Now I am getting the correct activity from the flutter plugin. But when I create bitmap image from activity.getWindow().getDecorView(), I am getting a all white blank image.

To create bitmap from activity, I am using Canvas

DecorView decorView = activity.getWindow().getDecorView();
Bitmap bitmap = Bitmap.createBitmap(decorView.getMeasuredWidth(), decorView.getMeasuredHeight(), Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(bitmap);
decorView.layout(0, 0, _window.getDecorView().getMeasuredWidth(), decorView.getMeasuredHeight());
decorView.draw(canvas);

So bitmap I am getting is blank, but my Flutter Activity has multiple widgets.

I tried to implement ActivityAware and used the override methods to get activity, but those methods didn't even execute.

How do I capture those?