mMediaUri is always null, even on the emulator

protected DialogInterface.OnClickListener mDialogListener =
            new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    switch (which){
                        case 0: //Take Picture
                            Intent takePhotoIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
                            // get path to save images
                            mMediaUri = getOutputMediaFileUri(MEDIA_TYPE_IMAGE);
                            if(mMediaUri==null){
                                //display an error
                                Toast.makeText(MainActivity.this,
                                        R.string.error_external_storage,
                                        Toast.LENGTH_LONG).show();
                            }else {
                                takePhotoIntent.putExtra(MediaStore.EXTRA_OUTPUT, mMediaUri);

                                startActivityForResult(takePhotoIntent, TAKE_PHOTO_REQUEST);
                            }
                            break;

                        case 1: //Take Video
                            break;

                        case 2: //Choose Picture
                            break;

                        case 3: //Choose Video
                            break;
                    }
                }
                private Uri getOutputMediaFileUri(int mediaType) {
                    //To be safe, you should check that the SDCard is mounted
                    //using Environment.getExternalStorageState() before doing this

                    if(isExternalStorageAvaible()){
                        //Get the URI

                        //1. Get the external storage directory
                        String appName = MainActivity.this.getString(R.string.app_name);
                        File mediaStorageDir = new File(
                                Environment.getExternalStoragePublicDirectory(
                                        Environment.DIRECTORY_PICTURES),
                                getString(R.string.app_name)
                        );

                        // create subdirectory
                        if ( ! mediaStorageDir.exists() ) {
                            if ( ! mediaStorageDir.mkdirs() ) {
                                Log.e(TAG, "Failed to create directory");
                                return null;
                            }
                        }
                        //3.Create a file name
                        File mediaFile;

                        //4. Create the file
                        Date now = new Date();
                        String timestamp = new SimpleDateFormat("yyyyMMdd_HHmmss", Locale.US).format(now);

                        String path = mediaStorageDir.getPath() + File.separator;

                        switch (mediaType) {
                            case MEDIA_TYPE_IMAGE:
                                mediaFile = new File(path + "IMG_" + timestamp + ".jpg");
                                break;
                            case MEDIA_TYPE_VIDEO:
                                mediaFile = new File(path + "VID_" + timestamp + ".mp4");
                                break;
                            default:
                                return null;
                        }

                        Log.d(TAG, "File: " + Uri.fromFile(mediaFile));

                        //5. Return the file's Uri
                        return Uri.fromFile(mediaFile);
                    }else {
                        return null;
                    }
                }

                private boolean isExternalStorageAvaible(){
                    String state = Environment.getExternalStorageState();
                    return state.equals(Environment.MEDIA_MOUNTED) ? true : false;
                }
            };