Black screenshot of the Unity app from Android

I made an android native plugin to take a screenshot. (Unity player 5.3 - Min version android API 19) I know it's possible to use Application.CaptureScreenshot or render a texture from Unity, but I really need to make my screenshot of my Unity app from android.

I created a java class with no heritage with the static method takeScreenshot :

     View screenView = UnityPlayer.currentActivity.getWindow().getDecorView().getRootView();
     screenView.setDrawingCacheEnabled(true);
     Bitmap bitmap = Bitmap.createBitmap(screenView.getDrawingCache());
     screenView.setDrawingCacheEnabled(false);
 
     Date timeStamp = new Date();
     android.text.format.DateFormat.format("yyyy-MM-dd_hh:mm:ss", timeStamp);
     String path = Environment.getExternalStorageDirectory().toString() + "/" + timeStamp + ".jpg";
     File imageFile = new File(path);
 
     FileOutputStream outputStream = new FileOutputStream(imageFile);
     int quality = 100;
     bitmap.compress(Bitmap.CompressFormat.JPEG, quality, outputStream);
     outputStream.flush();
     outputStream.close();

Then on Unity I have this onGUI :

     void OnGUI() {
         if (GUI.Button(new Rect(10, 10, 150, 100), "screenShot")){
             AndroidJavaObject jo = new AndroidJavaObject("MyScreenshotAndroidClass");
             jo.CallStatic("takeScreenshot");
         }
     }

I have a picture saved on the device, with the good size, but always black. I don't know if it's a problem when I get the unity view, a problem when I create the bitmap or a problem when I write the file.

I tried to replace the getDrawingCache by using the Canvas :

 Bitmap bitmap = Bitmap.createBitmap(screenView.getWidth(), screenView.getHeight(), Config.ARGB_8888);
 Canvas canvas = new Canvas(bitmap);
 screenView.draw(canvas);

But I had the same result.

Anyone have a clue to help me ?